Each segment in a 14-segment display character is labelled with a letter. The decimal point is a special beast; not only is it an unofficial 15th segment, it also gets two letters to describe it.
The map is as follows:
Note that this is exactly how they are lettered in the datasheet. This lettering is used throughout the software that drives the thing, most notably in the display font code. It’s all mere comments, but it makes the whole thing understandable.