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.