* all other slots to &80 + number
That's basic Plus cartridge behaviour. It doesn't fully decode RMR2 to set other exotic mappings as the [[Play2CPC ]] does. That's only required for it to serve as full external replacement for the internal Plus cartridge port, but as far as I (Abalore) know no software uses it but maybe the Burning Rubber + BASIC cartridge.
Programming of a cartridge game is just enable/disable lower and upper ROM when needed, and change slot with OUT &DF,n