Awfully hard to test our software on a new file system if I can't convert the boot drive to the new file system

Since you have space to spare: convert an empty partition to APFS, then install 10.13 on the APFS partition. Or use SuperDuper or somesuch to copy your existing 10.13 installation to the new partition.

There's an important distinction that needs to be made that's key to understanding the "rules." 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.