Build standby jobs like any other job (tick βπ Standby / fallback jobβ on Add Job) β they stay dormant and don't show on the board, scheduler or any engineer's list. Then pick one as each engineer's fallback below. The office is warned at 3:30pm & 6pm about anyone with no job for the next working day, and at 7pm each still-empty engineer's fallback is made live (Friday covers Monday). It appears to them at 5pm the evening before, and drops automatically if they get a real job.
Ticked people are left out of the 3:30pm/6pm βno job tomorrowβ warning and the 7pm auto-assign β e.g. apprentices, managers, or anyone not on the daily fallback cycle.