Spare cell
Beginner
Extra unused parts scattered around the chip, so a late bug fix can be made by changing only the wires.
Novice
Unused logic cells (inverters, NAND gates, flip-flops) scattered across the chip with their inputs tied to fixed values. If a bug turns up late, a fix can be built by rewiring nearby spares, which changes only the metal wiring layers.
Expert
Spread uniformly during placement so any late functional fix (a metal-only ECO) has spare gates nearby. They cost area and leakage. Too few spares near a patch means long ECO wires, which can cause timing violations and congestion.
Explained in Placement (Design Flow).