Most trucks in the capacity range are rated at 24" load center. Most manufacturers assume a 48" cube. The A usually shows the middle of a 48" load length which is 24" in this case. The B shows the middle of the load height which is usually assumed to be 24". The C is typically the lift height to the top of the forks. Some manufacturers measure the lift height to the bottom of the forks.
Back in the day, most Yale capacity plates showed the capacity of the truck to the center of the load. For example if the maximum fork height was 172", the C on the capacity plate would show 196". (172" + 24"). A lot of people mistook the 196" for the maximum fork height.
There are also dual capacity plates which either show trucks rated at 2 different load centers or 2 different lift heights.