They don't. Microchip assumes 255 bytes of ram will be available to store the data in. They start at address 0x10 (command byte) then if you look at No_DLE they store the data (this all happens after comparing the byte to STX,ETX,COMMAND, and DLE. (doing this from memory, so if I left any out, sry) The MOVLF INDF command stores the byte at 0x10 + FSR. at the end of the No_DLE routine, they have INCF FSR. this is how they are incrementing the ram address. They are using indirect addressing for all of this.

I am not sure if we have a simiular thing in PBP. Lookup does it, but that finds stuff, not store it.