What do the letters on the inside of the punnett square stand for

The inside of a punnet square tells you the possible combinations of genes the offspring of the parents on the outside can create.

Are you asking about a street map usage  if so here is your answer  a street map for a very limited area could simply be a bit map or a .jpg image.  but a more general map would probably store the streets as vectors.  but you shouldn't assume that there is one way to do it