To me, the wording seems to imply that these are bonuses that work even when you’re not using that particular car. Can anyone confirm this?
I’m talking about Car Mastery skills that say something to the effect of “you receive a permanent x% bonus to Event XP.” I’ve encountered it on several cars so far. Off the top of my head, two examples are the Sesto Elemento FE and the old Ford Transit van.
That just means you don’t have to unlock that Mastery again for that car. Some Masteries are only good for 10 races and you must unlock them again using skill points.
And all of them only work when you are driving that specific car, they don’t do anything when the car is sitting in the garage while you drive another one.
It just means that it won’t run out like the other perks that only work on 10 races at a time. That permanent bonus is there indefinitely, as long as you use that car in races. If you use any other car without the perk, you won’t get the bonus.
If that were the case, I would expect it to be worded like other bonuses that say ‘while driving this car you get x% bonus.’
That bonus probably means while in free roam.
I think there’s specifically a ‘while in free roam’ tagline for that, but now we’re getting into things I would have to double check, which I’m not in a position to do right now.
They only work when driving the car ortherwise you would see things like ultimate bonus @ 2000+ skill points, or even 10000+ which never happens even when you have a garage full of perked cars.
Permanent - applies while driving that car in free roam or events
Non-permanent - you ‘buy’ 10 uses and then you have to rebuy.
I haven’t seen a permanent bonus for skill points, just event XP I believe. I think there’s a category for bonus XP on the results screen it applies to. So it wouldn’t apply to Ultimate Bonuses and the like in any case.
I’d say just max out the skill trees for the cars you use the most and for those you use for XP or skill grinding, only worry about the perks on those instead. 
The wording is fine it lasts forever instead of 10 races. They could put Infinite I suppose.