De la Wikipedia:
"BMW uses a three-segment nomenclature for motorcycles. The first segment indicates the engine type, the second indicates the approximate engine displacement in cubic centimeters (with one notable exception: the F 650 GS, which has an engine displacement of 798cc[41]), while the third indicates the class of motorcycle (e.g., sport, sport touring, dual-sport, etc.). The three segments are separated by blanks.[42][43][44]
A similar 3-segment nomenclature is used for BMW automobiles. However, there is significantly less consistency across the range with respect to the second segment (engine displacement/100).
Engine type:
R – air-cooled, horizontally opposed ("boxer") 2-cylinder, oil-cooled 1994-2013, water-cooled after 2014
K – water-cooled, inline 3-, 4- or 6-cylinder
F – water-cooled, vertical 1-cylinder (through 2006), vertical 2-cylinder (after 2006)
G – water-cooled, vertical 1-cylinder
S – water-cooled, inline 4-cylinder superbike
Engine displacement in cc:
Current models: 1600, 1300, 1200, 1000, 900, 800, 650 and 450. Previous models included 850, 1100, and 1150.
Older model BMWs divide the approximate engine displacement by ten for the model number. For example, K75 ≈ 750 cc.
R1200RT-P authorities model
Styling suffix designations:
C – Cruiser
CS – Classic Sport
G/S – Gelände/Strasse Off-road/Street
GS – Gelände Sport Off-road Sport (Enduro)
GT – Gran Turismo or Grand Touring
LS – Luxury Sport
LT – Luxus Tourer (Luxury Tourer)
R – Road or Roadster, typically naked
RR – Superbike/Racing
RS – Originally stood for Rennsport, but since 1976 has stood for Reisesport[45]
RT – Reise Tourer (Travel Tourer)
S – Sport
ST – Strasse (Street) or Sport Tourer
T – Touring
Additionally, a bike may have the following modifiers in its name:
A – ABS
L – luxury
P – police
C – custom
PD – Paris Dakar
Examples: K 1200 S, R 1200 RT, F 650 GS, R 1150 RSL, K 1200 LT, K 1200 LT-C, R 1200 RT-P, R 1200 RSA, S 1000 RR.
Prior to the introduction of the K 100 series and the R 1100 series motorcycles, the letter prefix was always the same, and the numbers were either based on displacement, as mentioned above, or were just model numbers."