Neither. It is an accessory that mounts underneath the tractor itself. You can see a version of it here:
It is an incredibly powerful unit. I ran over rebar that was vertically in the dirt like a post that was one inch thick and the thing sheered it off without me even knowing it! Sadly, it also tore off the sprinkler that was next to it also.
Yeah the Bushhog will eat anything you can get under it. Saplings are a specialty.