Not used. Defines a mapping between two regular expressions. One expression is used to find a matching string in the message string and the other is used to alter the string before the service places it back in the message string.
The symbol to use to reference the mappings in your application. The Message Compiler (MC.exe) uses the symbol to create a constant for the map in the header file that the compiler generates. If you do not specify a symbol, the compiler generates one for you.