When macOS High Sierra is released to the public next week, the new Apple File System (APFS) feature will be limited to Macs with all-flash built-in storage, which means it won’t work with iMacs that include Fusion Drives.

iMacs with Fusion Drives were converted to APFS during the beta testing process in the first macOS High Sierra beta, but support was removed in subsequent betas and not reimplemented.