Note. In order to distinguish bogie wagons, the last letter of the main class was doubled, e.g. G became GG. Unlike passenger stock, for goods wagons the doubling of letters was retained. Only on Dgw and ZM wagons was the differentiation dropped, whilst the BT, H, Pwg, V and VO wagons had no bogie variants anyway.
Note. There are several pages on Wikipedia in more depth.

Primary Classification Letters

BT from ? Groß Behälter-Tragwagen. Large container-carrying flat wagon.
Dgw from ? (DB) Dienstgüterwagen. Works wagon.
G Gedeckter Güterwagen. Covered van.
HH to 1923 Langholzwagenpaar. Log wagon pair.
H from 1924 Cradle wagon. Type of Langholzwagen or log wagon.
K Klappdeckelwagen. Lidded wagon.
M from about 1957 (DR) Militärwagen? Military van - a covered van with special equipment for carrying troops
N 1911 to 1921 Covered van or combination van with air brakes or pipes for use in fast trains
O Offener Güterwagen. Open wagons.
Pwg from 1933 Gepäckwagen für Güterzüge. (Luggage van for goods trains.) Guards van.
R from 1914 Rungenwagen. Stake wagon.
S Schienenwagen. Flat wagon (e.g. for carrying rails).
T from 1943 Thermoswagen. Refrigerator van.
V Verschlagwagen. Livestock van for small livestock e.g. sheep and poultry.
VO to 1921 Offener Viehwagen. Open livestock wagon for large livestock, e.g. cattle and horses.
X Open works wagon.
Z from 1951 (DR) Zisternenwagen. Tank wagon.
ZM 1945 to 1950 Tank wagon for the transportation of crude oil.

 Secondary letters for Goods Wagons

Used with classes Relevant Period Meaning
m all to 1910 At least (mindestens) 15t maximum load
1911 to 1923 At least 15t but less than 20t maximum load
mm all At least 20t maximum load
m G, H, K, O, R, S, V, X ab 1924 At least (from 1937 exactly) 20t maximum load
mm G, K, O, R, X from 1937 more than 20t maximum load
w all to 1923 Less than (weniger) 10t maximum load
G, H, O, S, T, V, X fro 1924 Less than 15t maximum load and not tippable

 Wagons with eight wheels or more (including HH wagons)

Used with Classes Relevant Period Meaning
m GG, HH, OO, SS to 1910 At least (mindestens) 20t maximum load
1911 to 1923 At least 30t maximum load
SS 1933 to 1950 At least 50t maximum load (omitted on SSt and SSy, see below)
from 1950 (DB) At least 35t maximum load (omitted on SSt and SSy, see below)
w GG, HH, OO, SS to 1923 Less than (weniger) 20t maximum load
GG, KK, OO from 1924 Less than 30t maximum load
SS Less than 35t maximum load

 Description of loading length and loading area

for Classes Period valid Meaning
k S from 1924 Loading length shorter (kürzer) than 13m
SS Loading length shorter than 15m
l all except SS to 1910 At least 7m loading length (Ladelänge) and 17m² loading area
all except R and SS 1911 to 1923 At least 8m loading length
SS to 1923 More than 12m loading length (Ladelänge)
1924 to 1932 More than 15m loading length (omitted on SSt und SSy, see below)
from 1933 More than 18m loading length (omitted on SSt und SSy, see below)
G 1924 to 1932 At least 24m² loading area (Ladefläche)
from 1933 At least 26m² loading area
K from 1960 (DB) At least 33m² loading area
O from 1951 At least 10m loading length (Ladelänge)
X from 1951 (DB) At least 8m loading length

 Description of features of importance for train formation

  for Classes Relevant Period Meaning
b G, R, T from 1933 Ferry wagons (Fährbootwagen) for German-English ferry services (with smaller loading gauge)
e all from 1924 Electric heating pipes (elektrischer Heizleitung)
ee all from 1933
(DR only to 1950)
Electric heating pipes (elektrischer Heizleitung) and electric heating (elektrischer Heizung)
f all 1924 to 1932 Ferry wagons (Fährbootwagen) for German-English ferry services (with smaller loading gauge)
h all except ZM from 1924 Steam heating pipes (heizleitung)
hh G, GG, T from 1933 Steam heating pipes (heizleitung) and steam heating (Dampfheizung)
n G, V, VO to 1923 Air brakes or air pipes. Steam pipes or other equipment that make it suitable for use as a luggage van or in passenger trains (from 1911 some formed the main letter class N, see above)
q all from 1951 (DR) no through couplings (0-0 wagon or 'Null-Null-Wagen'), maximum load reduced
r all from 1924 Wagon with exchangeable wheelsets for transiting to Russian (russische) broad gauge (omitted on SSy, see below)
s G, R, SSy, T 1933 to 1950 Suitable for fast (schnell) trains running at up to 90km/h
GG, RR, TT Suitable for trains running at up to 120km/h
G, R, SSy, T from 1951 (DR) Suitable for trains running at up to 100km/h
GG, TT Suitable for trains running at up to 120km/h
alle ab 1951 (DB) Suitable for trains running at up to 100km/h
u X to 1914 Unsuitable (untauglich) for trains used as public transport
[u] O, HH, S/SS, X to 1921 Unusable (unbrauchbar) for the loading of military vehicles
u alle 1939 bis 1948 Unusuitable (ungeeignet) for military transportation
G ab 1948 (DB) Unusuitable (ungeeignet) for passenger transportation
O sides not removable? ('nicht abbordbar')

 Descriptions for specific wagon classes

 Specific Descriptions for Classes G/GG and N

  Relevant Period Meaning
f to 1923 Stalls for race horses (Luxuspferde)
g from 1951 (DB) Floor hatches for unloading bulk material
i 1911 to 1927 (?) End platforms, suitable for passengers
k 1924 to 1944 Refrigerated (Kühleinrichtung') (from 1945 several formed the main letter class T, see above)
from 1951 (DR) Refrigerated van (Kühlwagen), old type
ll from 1933 Pair of close-coupled vans for part-load goods, known as a Leig-Einheit
p from 1924
(DB only to 1950)
Six-wheeled, dead weight 16t
t from 1924 End doors (Stirnwandtüren)
trieb from 1933 to 1960 Goods railbuses (Gütertriebwagen) (all were left with the DB after the war)
v 1924 to 1932 End loading doors and special ventilation for livestock transportation (Viehbeförderung)
from 1933 Stable wagon with groom compartment
x from 1951 (DR) Fixed (internal?) walls for grain transportation

 Specific Descriptions for Class H/HH

  for Class Relevant Period Meaning
o H from 1924
(DR only to 1950)
Without (ohne) stakes, only usable in pairs
s H/HH to 1950 Coupling rods (Kuppelstangen)
z HH to 1923 Prongs (Zinken) on the rotating cradles

 Specific Descriptions for Classes K/KK, O/OO, VO and X/XX

  for Classes Relevant Period Meaning
c O to 1910 Coke wagon (Cocswagen) with high sides in order to load the right amount of coke corresponding to the maximum load
from 1911 Maximum 15t load, wooden sides at least 1,30m high but lower than 1,90m
f O/OO from 1951 Rotating and folding drop ends for transporting vehicles (Fahrzeugbeförderung)
g O 1924 to 1950 High, fixed rail sides (Gatterwänden)
g K from 1957 (DB) Sliding sides
i O/OO from 1951 (DB) Tippler (Muldenkippwagen)
k O/OO to 1923 Front walls (Kopfwänden) that pivot about an upper hinge and allow the load to be dumped onto tipping equipment
k O from 1933 Bucket car (Kübelwagen) with two or three removable buckets, 25t load
OO from 1951 (DB) Bucket car with four or five removable buckets
from 1951 (DR) Transporter for large containers
k K from 1951 (DB) Crane (kranbar), sliding roof with crane loading and unloading
n O/OO from 1951 (DR) Low (niedrige) sides, 40-80cm high
o X/XX from 1914
(DR only to 1950)
Sides over 40cm high
p O/OO from 1924 Not tippable, at least 15t load, sides up to 1,90m high
q O/OO to 1923 Removable end walls
t K/KK, O/OO,
Funnels (Trichtern), floor hatches, inclined floors or side hatches for self-discharging, not tippable. X wagons additionally with removable end walls
v O from 1924 Wooden sides over 190cm high, no tipping facility (ex main class VO, see above)
y X 1914 to 1923 Special equipment for rapid loading and unloading etc.
z O, VO to 1923 Specially sealed, and so suitable for peat litter transportation or the like
O/OO from 1933 Ore transportation (Erzbeförderung) with special sealing

 Specific Descriptions for Classes R/RR and S/SS (Now Class U.)

  for Classes Relevant Period Meaning
a SS from 1933 Open brakeman's platform, folding platform rails
o R from 1933 No stakes
Ru Ol, Sl 1911 to 1913 At least 10m long and with stakes (formed separate main letter class R from 1914, see above)
t S/SS from 1924 Low-loader (Tiefladewagen).
y SS from 1942 Heavy goods flat wagon (Schwerlastwagen) with 50t maximum load, loading length 8.8m (with folded platform rails 9.5m), open brakemans' platform, exchangeable wheelsets for broad gauge
yl RR
from 1951 (DR)
from 1951 (DB)
As for y, but longer loading length
ym SS from 1942 As for y, but higher maximum load (DR: >50t, DB from 1951: 80t)
RR from 1951 (DR) Heavy goods flat wagon with 80t maximum load, loading length 14.36m, open brakemans' platform, folding platform rail

 Specific Descriptions for Class T/TT

  Relevant Period Meaning
f ab 1943 Fish (Seefische) van only
g Refrigerated goods (Gefriergut) only
gg Refrigerated goods (Gefriergut) and dry ice only
k from 1943 (DR) Refrigerator van (Kühlmaschinenwagen)
from 1951 (DB) k, an insulation value
kk from 1951 (DB) Insulated milk can van
n ab 1943 Not (nicht) suitable for refrigerated goods
o No (ohne) meat hooks
v Electric ventilators (Ventilatoren)

 Specific Descriptions for Class V

  Relevant Period Meaning
e to 1910 with decks (Etagen)
1911 to 1921 Lattice walls and two floors
g 1911 to 1932 Intermediate floors for geese (Gänsebeförderung) (to 1921 always used in combination with e)
from 1933 Four floors
p from 1922 No equipment to alter the loading area
z to 1921 With equipment to alter the loading area

 Specific Descriptions for Class ZM

  Relevant Period Meaning
h 1945 to 1950 Heating pipes (Heizschlangen)
i Insulation (Isolierung)
s For black (schwarzer) crude oil
w For white (weisser) crude oil

 Specific Descriptions for Class Z/ZZ

  Relevant Period Meaning
d ab 1951 (DR) Heating pipes or tank
i Interior (innerer) lining
w Only for fuel

 Bavarian goods wagon classes

 Epoch Ia - Early (1867-1892) wagon classification system

AGoods Vans
B/BDBahndienstwagen – Flat wagons for MOW (Maintenance-of-Way) work
CHeizwagen – Vans with steam boilers for train heating
D/DWDienstwagen – Guards vans for P and G trains
EOpen wagons with high sides
FOpen wagons with low sides, long
GOpen wagons with low sides, short
HTimber trucks
JGepäckwagen – Luggage vans without guards compt.
KDouble-deck livestock vans
LLow-sided open wagons for stone transport
MPeat wagons
NHorse vans
OCattle wagons, covered or open

 Epoch Ib - Middle period (1893-1912) wagon Primary and Secondary classification and numbering systems

 Meaning  Meaning
VViehwagen – Livestock wagons (19001 – 20000) l, pfor long and very long vehicles
GGedeckte Güterwagen – Goods vans (30001 – 50000) d, hfor low and high sides
OOffene Güterwagen – Open wagons (50001 – 70000) w, mfor < and > 10t max load respectively
HHolzwagen – Timber trucks (70001 – 76000) e, f,
y, etc.
for special features of certain wagon types
SSchienenwagen – Platform wagons (76001 – 80000)
X Special wagons, railway-owned / Special wagons, private-owned / Wagons of private railways

 Epoch Ic - End period (1912-1920) wagon classification system

GGedeckte Güterwagen – Goods vans (30001 – 44000)
NHorse vans (44 001–48 000)
V & KViehwagen – Livestock wagons, single & double deck 48 001 – 50 000
OO wagons 50 001 – 70 000
HH wagons 70 001 – 76 000
RR wagons 76 001 – 78 000
SS wagons 78 001 – 81 000
Special wagons 81 001 – 82 000
XX wagons 82 001 – 87 000
Private railway wagons 90 001 – 92 000
Railway tank wagons 500 000 – 501 999
Private tank wagons 502 000 – 504 999
Other private wagons 600 000 – 602 999
Railway works wagons 700 001 – 702 000