Well, it is the choice of what the installer has on the truck.
Probably an HR21, but maybe an HR22 or HR23. Functionally they are all the same except the HR22 and HR23 have larger hard drives.
The HR23 also has different hardware inside, different chipsets etc, its an all around better receiver, but it has its problems imo. it depends entirely upon what the tech has on his truck, to tell you the truth my hsp just got a stock of refurb h20s last month and we were told install them for new customers.