SP is an abbreviation for Support Processor. It is the separate processor that control the IO system functions, for example. the hard disks, optical disks, floppy disks and all the Alpha numemric (AT) terminals, later called Local Craft Terminals (LCT's) and all other communication links between the AXE and humans or other computer systems. i.e. X25/V36 protocol links.
Several variants of the SP-based IO Systems existed including IOG 11A, IOG 11B, IOG 11C, IOG 11B5, IOG 11C5 and IOMC.