A stack plan (or stacking plan) is a floor-by-floor view of which teams and functions occupy which parts of a building. It is the planning tool for big allocation questions: which floors to keep, which teams to co-locate, what to do when a lease event approaches.
Hybrid work turned static stack plans into living documents. When attendance swings by day, the stack needs utilisation data behind it, otherwise you are stacking headcount that never shows up on the same day.