Mac storage basics · Purgeable space and local snapshots
What is purgeable space on a Mac?
Short answer: Space macOS is using but will free automatically when needed — mostly local snapshots, downloaded iCloud copies and some caches. Finder counts it as available.
Purgeable space is storage macOS is using right now but has marked as safe to remove automatically when something needs the room — local Time Machine snapshots, downloaded copies of iCloud files that are also in the cloud, and certain caches.
Finder counts it as available, which is why free space can look healthy while an installer still complains. macOS frees it on demand, but not always quickly enough for a big update or copy.
You can speed it up by releasing local snapshots or making cloud files online-only. Storage Studio's Local Snapshots window and Optimise Cloud Storage Locally do both, and its free-space figures say how much is really free versus held. See local snapshots.