What determines whether a drive can be converted to APFS? Now High Sierra is released and not in beta, I'm wondering what the rules are about covering drives to APFS.

The release version of High Sierra auto converts SSDs to APFS on installation. But we know that Apple pulled the conversion of Fusion drives during the beta. We also know that non-fusion spinning disks can be converted to APFS but are not auto-converted. 

According to the developer advice and several internet sources, other drives can be converted by using Disk Utility (right click the disk and the option will be greyed out if not available). 

So I thought I'd try this on an old (late 2009) 27" iMac and the option was not available (the internal disk is actually new this year so it isn't just age). My Macbook Pro (early 2015) internal SSD converted automatically. Then I have some external disks all of which are perfectly functional. So I tried some of them. My Time Machine external disk would convert (not sure whether it is worth it) and old spinning 500GB spinning disk (extracted from an old MacBook) will convert. But two other disks (a LaCie 2GB USB and the original 1GB from my iMac) will not.

So what are the rules? Which disks can be converted to APFS? It is clearly more complicated that Apple's guidance which implies any HFS+ drive can be converted. Is it a settings thing we can manually alter, or is it a hardware thing we can't fix?
There's an important distinction that needs to be made that's key to understanding the &quot;rules.&quot;  There's:

Supporting APFS
Converting to APFS

Any drive, spinning (HDD) or solid state (SSD) will support APFS. You can wipe it clean and do a new partition with an APFS volume.
Apple File System Guide FAQ:

Can I use Apple File System with my existing hard disk drive?
Yes. Apple File System is optimized for Flash/SSD storage, but can also be used with traditional hard disk drives (HDD) and external, direct-attached storage.

Paraphrased from the FAQ:

Apple File System is a new, modern file system for...macOS....It is
optimized for Flash/SSD storage...
APFS replaces HFS+ as the default file system for...macOS High Sierra and later.

Converting (during install or otherwise) is another story.

Installs/Upgrades with an SSD will convert automatically.
Installs/Upgrades with an HDD will not.
You cannot convert non-Mac volumes at all. Though you can erase the drive and do a fresh install with APFS.

So as a use case, you install macOS High Sierra on a iMac with a single HDD. It will not convert, and it won't give you the option because you cannot convert a boot volume. You can't even erase a boot volume for that matter. However, if you boot into macOS Recovery, you will have the ability to convert your drive.
If you have a USB external formatted as HFS+, it will convert.  If it's formatted FAT32 (for example), it won't; you can, however  wipe it and format it with APFS.