Answer
ZBB suits discretionary and support/overhead costs (e.g., marketing, admin, R&D, public-sector services) where spending isn't directly tied to output and needs justification. It's less suitable for essential, volume-driven production costs (direct materials/labor) that vary predictably with output, where standard/flexible budgeting is more efficient.
Real-world example ZBB is applied to overheads and support functions, while direct production costs use standard costing.