Apple File System (APFS), the default file system for Mac computers with solid state drives, features strong encryption, space sharing, snapshots, fast directory sizing, and improved file system fundamentals. While APFS is optimized for the Flash/SSD storage used in recent Mac computers, it can also be used with older systems with traditional hard disk drives (HDD) and external, direct-attached storage. macOS High Sierra supports APFS for both bootable and data volumes.
The APFS file system allocates disk space on demand. When a single APFS container (partition) has multiple volumes, the container’s free space is shared and can be allocated to any of the individual volumes as needed. Each volume uses only part of the overall container so the available space is the total size of the container, minus the size of all volumes in the container.
To learn more about APFS, see the Apple File System Guide.