Apple File System (APFS) is the default file system in macOS High Sierra for Mac computers with all-flash storage. APFS features strong encryption, space sharing, snapshots, fast directory sizing, and improved file system fundamentals.

When you install macOS High Sierra on the built-in solid-state drive (SSD) of a Mac, that drive is automatically converted to APFS. Fusion Drives and hard disk drives (HDDs) aren't converted. You can't opt out of the transition to APFS.

APFS compatibility

Devices formatted as Mac OS Extended (HFS+) can be read from and written to by devices formatted as APFS.

Devices formatted as APFS can be read from and written to by:

• Other devices formatted as APFS
• Devices formatted as Mac OS Extended, if using macOS High Sierra

For example, a USB storage device formatted as APFS can be read by a Mac using High Sierra, but not by a Mac using Sierra or earlier.

APFS and FileVault

FileVault volumes are converted from Mac OS Extended to APFS, just like unencrypted volumes. 

APFS and Boot Camp

Boot Camp doesn't read from or write to APFS-formatted volumes, but is compatible with High Sierra.

APFS and file sharing

• Volumes formatted as APFS can't offer share points over the network using AFP. 
• APFS supports SMB and NFS, with the option to enforce only SMB-encrypted share points.

APFS and Time Machine

• You don't need to change any Time Machine settings to back up APFS-formatted disks.
• Any Time Machine share points must be shared over SMB instead of AFP.