Best way to create a Sierra partition on High Sierra APFS SSD?

In order to install Sierra, the partition must be HFS+ (Mac OS Extended). It was possible during Sierra's beta testing (and afterwards if you wanted) to make the volume it's on APFS. But, APFS itself was still considered beta throughout Sierra and was only intended to be used by those who wanted to fully test APFS ahead of the official specification release in High Sierra.

That said, rule numero uno! Backup before proceeding!

As you'll see next, it's really easy to bork your drive with Disk Utility. It always has been as partitioning is a direct modification of the drive layout, but it seems even more so now. This is going to be lengthy (mostly screen shots) because it's the only way to show how difficult this can be to do.

I have to split the response since the forum thinks 12 images is too many for one post.

My preferred way of creating partitions was to do it from another bootable drive. Seems logical since you can then do whatever you want to another physical drive without trying to work around the volume you're currently booted to on the same drive. Nope. Not any more. Here's what happens.

I have two physical drives with High Sierra on. My SSD, and a hard drive (both in a 2010 Mac Pro tower). I booted to the hard drive and went to try dividing the SSD. Notice that the High Sierra volume I'm booted to has the Finder icon over the drive icon.

The SSD does not.

Why does this matter? Because you literally cannot partition the non-Finder marked drive. Not they way you want to, that is. When you first click the Partition button, you get this sheet. Unless you check the box, you'll see it every time.

Adding an APFS shared volume isn't what I want. And besides, you can only choose another APFS type when you do that.

So I click Partition. Notice on the non-Finder tagged APFS drive, you cannot click the +/- buttons.

As seen, I can select any format type I want, but when you do that, it doesn't create a partition, it is going to wipe out the entire drive (next image)! It does this no matter what type of format you choose.

Okay, so what if you try these steps on the APFS drive you're booted to? The one that has the Finder icon over the drive icon?

(Quick note. The hash lines show where data is on each partition.) Anyway, now the + button is active on the Finder "approved" drive. Or whatever that means. I can split the APFS partition no problem by highlighting it and clicking the + button.

I can also do that to the Files partition (which is HFS+) and make the new split APFS, HFS+, or other.

So, knowing that, I booted to the SSD with High Sierra on it. Now that drive is "Finder" approved, and the volume, APFS Repair, isn't.

But I can now easily split it (the button can of course be moved if you don't want exact halves), and I can format it as HFS+ while leaving the High Sierra side as APFS.

FWIW, From reading the page I linked it doesn't seem possible to create a HFS+ partition on a APFS formatted drive. So I think your options are, reformat your internal as HFS+ and install Sierra, create a partition and install High Sierra on that. Or, what I would do, just install Sierra on an external drive formatted as HFS+.

Did you read this?

Partition a physical disk using Disk Utility on Mac - Apple Support