In the FDNY they are different things. Reserves (500’s for Engines, 600 for the Tower Ladder, 700’s for Ladders, and 800 for the Squad) are equipped and basically ready to go into service if needed in moments.
Spares are completely empty and when a company needs to move to a spare they must transfer all equipment from their regular unit.
Reserves are typically used for extra staffing during special events or sometimes storms I think. Spares are for when a company regular unit is going to the shops or is unavailable for a period of time.