Spare usually means not being booked to a fix turn and is, subject to T&Cs, effectively "as required". You sit around unless you're told to do something.
As required might be a booked diagram in itself for example a standby cover turn or for drivers a fleet standby to do inter depot moves, test runs etc. It can also be a chunk of a diagram where you might work to a location to then be used as required - for example take a unit to a depot on a booked move, work to the directions of the supervisor as required for 5 hours and then take a booked move back out again, or a shed or ferry turn.