It is not recommended that you use APFS on an HDD or fusion drive. You can install to an HFS+ partition, and are able to convert it later to APFS using Disk Utility. That only works, though, if installing from a USB drive. If you're installing by running the installer app from within another install of macOS (on a supported machine, of course), it will automatically convert your machine to APFS if it has an SSD. To download the full release when it comes out, you can use the App Store on a supported machine, or the built-in downloading feature of my macOS High Sierra Patcher tool if you're using an unsupported machine.Apple's website does actually specify that it will work on HDD but isn't automatically converted. It is simply optimised for SSD. Does that mean HDD users lose some new features along with other obvious benefits - possibly. @dosdude1 will be able to correct me if I'm wrong but from what I gather, you can upgrade to HS, then use disk utility to convert your HDD to APFS and keep your data, reverting back to HFS+ will cause you to lose your data.
Also, if you do update to APFS on a HDD on an unsupported mac, your computer will boot in verbose mode where you will see a black screen with all the boot commands and processes running in white text.
[doublepost=1505862866][/doublepost]@dosdude1 whilst we are on the topic, come Monday 25th on official release date, will the download link on your website provide a full final release version of HS or will it be a few days till that is up?
/Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/startosinstall --converttoapfs NO
Can I convert external HDDs?
Yes, after High Sierra install open diskutility, right click the external drive and click "Convert to APFS" (only works on HFS+ partitions). Now whether converting HDDs is useful is a whole other topic of discussion.