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 preceeding | (Also called "RSX slash").
Some of the best known RSX commands are the ones provided by AMSDOS, as they were present on all CPC664, CPC6128, 6128plus and CPC464 with DDI-1:
|A
Makes drive A: the current work drive
|B
Makes drive B: the current work drive
|CPM
Starts CP/M (or other systems) from a system disk
|DISC
Switches back to disk drive use after |TAPE
|TAPE
Makes the CPC use the tape recorder for file access
RSX command extensions can be implemented through ROMs (like AMSDOS or other disk operating systems) or by programs.