What's weird, and I'm not sure if it's a documented or undocumented feature, but the account I am logged into on the App Store differs from the one logged into on the system. The system Apple ID is setup with Family Sharing, and the users are able to use apps purchased with the secondary Apple ID.

I haven't transferred the purchases or anything either. The two Apple IDs have different purchases on them, and those on Family Sharing are able to access both.

Interesting. But WTF is a "primary" Apple account? My original Apple ID isn't an E-mail address, so they forced me (and others in that situation) to create another one for iCloud because that one inexplicably has to be an E-mail address.

I use both for quite a few things. Which one is "primary?"

That text is badly written. They define that after mentioning it:

“At the time of migration, the Apple Account signed in for use with iCloud and most features on your iPhone or iPad will be referred to as the primary Apple Account.

At the time of migration, the Apple Account signed in just for use with Media & Purchases will be referred to as the secondary Apple Account.”

⇒ apparently you can be signed into multiple accounts at the same time ¿but I guess with only one account per feature?

But as I said, that page is badly written. So, maybe I’m understanding it wrong.

Thanks for the clarification.

Yes... because of the mess Apple made, I am always signed into two accounts. My non-E-mail one for all purchases, and the E-mail one for iCloud.

It's not super difficult if you have an Apple ID from many years ago that you bought media with and then have a different Apple ID that you use for everything. Which isn't that uncommon for anyone who used iTunes and bought music or media and then forgot their ID and just made a new one when they got a iPhone or Macbook. Was able to transfer all my purchases to my main account pretty easily.

The real downside is if you have two fully active Apple IDs. Then things like calendars, photos, email, etc are still stuck on the other account until you export it. Which can be a pain since you have to sign out of your main account, sign into the old account and export, then sign back into the main account.