New file system Apple File System will be available only on a Mac with flash drives. Computers equipped with a hybrid system drives Fusion Drive, will continue to work on HFS+.
Fusion Drive is a data storage system that combines a flash drive (SSD) and magnetic hard disk drive (HDD) into one logical volume, allowing you to use them as a single drive. On the basis of this technology some models of iMac and Mac mini 2012.
During beta testing of the High Sierra macOS computers with a Fusion Drive has received the support of APFS, but after a few test releases, they lost it. After the release of macOS High Sierra GM Apple confirms that the new file system is only available on Mac c SSD.
Those who participated in beta testing and has managed to convert their drives in APFS, you will have to return to the previous file system HFS+. Otherwise, users run the risk of losing some data. For them, Apple has published guidelines for the preparation of a Fusion Drive to install macOS 10.13.
Apple File System is a new, safer and more modern file system optimized for flash drives. It will be available after the official release of macOS High Sierra on September 25.