Getting in what car?

If you're Hertz Gold, you get to choose any car in the Gold line (if you don't reserve the very lowest class cars, I think).

But if you're Avis Preferred, these days they simply assign you a car (they used have equivalent Select & Go, but it seems to be mostly gone now).

So if you just want to get in your assigned car and go, lots of companies can do that.

But if you want to be able to pick a car yourself and then go, you have to choose a car company that allows that like Hertz does.

And thus you have to specify which of these you want if you're coming from Hertz, which does both.