Right place to look, so probably the wrong interpretation of it.
In the current package structure, the unlimited package is indeed unlimited. No usage allowance, and no rate limits.
The prioritisation mechanism is in place to make sure that the time-sensitive protocols (as mentioned by RobertoS) get priority over "bulk" protocols that aren't time-sensitive - and this works in two separate ways...
If your own link is max'ed-out, then it ensures that the time-sensitive stuff is carried first, and the P2P-type stuff gets to fill the rest of the pipe.
If Plusnet's links are max'ed-out, then it ensures they get carried with priority there too. However, PN say they are careful to keep ahead of the bandwidth requirements on those links, so this part really only applies when faults happen somewhere.
IMO this prioritisation scheme is a good thing.
I have gone with BT now as I liked what they offered. But I see that you can get the add on for totally unlimited on Plusnet. For future reference, are they very good with the add on?
With the unlimited package, the "Pro" add-on is not necessary to overcome any rate limits, and stops this priority mechanism from working. It should only ever be needed on legacy products where rate limits were in place.
On those legacy products, it works fine.
The BT option isn't too bad either - especially if you want to make use of the TV option, or the BT Sport channel.
If they gradually knock together a better TV offering, I might get tempted to switch one day.
Heard BT own them?
Yes. That ownership doesn't seem to get in the way of day-to-day operations, but it probably does shape their strategies.