OK... another newb question but one I've tried to search the answer of...

I've seen the 'B#' designation in reference to certain vehicles but for the life of me I don't understand what is stands for... I assume it is related to model and year but I've not seens a 'chart' and I'm one of those 'visual type' people... Some guidence please...?

B5, B6, B7, I12, O57... BINGO!