Page 2 of that manual says "built-in production test mode", but I don't see anything else in the manual about that. But bitsavers also has a data book with S2000 info that has more detail:

The ROMS input is tied high for internal ROM only, low for internal and external ROM, but can also be tied to the SYNC output to "override bank 0 with an external program" or tied to inverted SYNC to "verify internal ROM contents". That sounds promising!