The symbol of a station is, in commonly used APRS trackers, typically configured using a pair of two characters. The first one selects the symbol table: primary "/", or secondary "\". The second character selects which symbol out of the selected table is displayed.
For example, a symbol configuration of "/>" selects the primary table "/" and the car symbol out of that table ">". A symbol configuration of "\L" selects the secondary table "\" and the lighthouse symbol from the secondary table "L".
The symbols in the secondary table can have an alphanumeric overlay. The overlay character can be a digit (0 ... 9) or an upper-case ASCII letter (A ... Z). The overlay character is placed in the symbol code's table selecting character in the place of the backslash "\".
For example, a symbol configuration of "N#" will display a green digipeater star with a N character on top.
It is important to understand that the symbol graphics are not transmitted over the air. Instead, just the two-character table-and-symbol code is transmitted, and the graphics are already stored at the receiving station. Each station receiving the position packets need to use the same updated symbol graphics files to see the same symbols for each station. The definitions and graphics of the symbols have been updated over the years, so it is a good idea to make sure you have the updated set.
The symbol graphics currently widely used can be downloaded at WA8LMF's site: