TidyMacMac storage glossary
The words macOS uses, in plain English.
System DataThe storage category macOS uses for everything that isn’t Apps, Documents, Photos or another named category.Purgeable spaceStorage macOS counts as available because it promises to free it when needed.Local snapshot (APFS)A read-only, point-in-time copy of a volume kept on the same disk.APFS containerThe pool of space that all the volumes on an Apple file system disk share.Sealed system volumeThe read-only, cryptographically sealed volume that holds macOS itself.Full Disk AccessA macOS privacy permission that lets an app read protected folders such as Mail, Messages and backups.Swap (virtual memory)Disk space macOS uses as extra memory when RAM is full.App containerA private folder in ~/Library/Containers where a sandboxed app keeps its data.