Crazy abbreviations

[Master]

Hello, when searching for parts, accessories and other stuff I often encounter abbreviations like these:
BMW 3/1 3C 3/C
I understand it has something to do with body type/model, but can someone explain what exactly does it mean?

Also I was looking for safety belts, and some sites say that I have to attach it to C-point or D-point, can someone explain where exactly these points are?

Thanks