It goes back to the standards of film. ASA (modern ISO) ratings were given to film and a doubling of the number meant doubling of the film's light sensitivity.
If 100 ASA colour or 125 black-and white wasn't sensitive enough, the photographer could choose 400 (it was common to, "Push," the latter (under-expose and over-develop) to 800 or even 1600 if conditions warranted) 200 ASA colour film was a later addition, giving better grain than 400, but better speed than 100. Fuji actually made a 1600 ASA colour print film at one point, and 400 ASA slide film was often pushed as well (the opposite of pushing is cutting, but this was seldom used).
The FSU used a broadly similar GOST system, while the Germans used the logarithmic DIN system, which added 3 to the number instead of doubling it. German cameras (and meters) are often calibrated in both ASA and DIN, while FSU kit will be in GOST and DIN.