That you are buying a bundle and it doesnt matter how much of the bundle you use you pay the same amount every billing period?

So if I buy entry to the swimming hall that allows me to be there for 4 hours but also allows me to leave earlier you would call that a flat rate?

I have never noticed there are people who interpret it that way.

If it allows you 4 hours total per month yea

That you are charged a single fixed fee regardless of usage.

Nothing about that prevents a usage cap.

A cap pretty much is the opposite of regardless of usage