The letters BCI stand for Battery Council International. The BCI number tells you the best battery size according to the physical dimensions, and the types of terminals required for the car that you own.
BCI assigns numbers and alphabetical lettering to batteries to show what group size they fit in. Some cars can actually accommodate batteries from one or more group sizes. The BCI should lead you to a battery with enough cranking amps and that fits in the battery compartment of your vehicle so it can be strapped down safely.