RSX (Resident System eXtensions) were a way to extend the command set of Locomotive BASIC, e.g. for accessing external peripherals like disk drives, light pens, speech synthesizers and so on.
RSX commands can be distinguished from normal commands by the preceding pipe symbol "|" (also called "RSX bar", "RSX pipe" or "RSX slash").
Some of the best known RSX commands are the ones provided by AMSDOS, as they were present on all CPC664, CPC6128, CPC6128+ and CPC464 with DDI-1:
Makes drive A: the current work drive
Makes drive B: the current work drive
Starts CP/M (or other systems) from a system disk
Switches back to disk drive use after |TAPE
Makes the CPC use the tape recorder for file access
For a full command reference to the AMSDOS RSX commands, see AMSDOS.
RSX command extensions can be implemented through ROMs (like AMSDOS or other disk operating systems) or by programs.
Reference documentation to write a RSX (assembly-level calls to register one, etc) is supplied in Soft968: CPC 464/664/6128 Firmware section 10.