B/RB engines? can somebody explain this to me?

i keep seeing B/RB or B or just RB for engine models/series, but im not sure which engine is which, and which one my 318 would qualify as (if either.) im guessing the 273, 318 and 340 are A engines, or maybe just the 273/318, and 360/383 are B engines and everything higher is an RB engine?
can somebody please rub some knowledge on my brain please :)