Depends on how you look at it. It keeps the whole house water pressure down. But the water pressure it is dealing with is called by the water heater heating cold water, which expands.The other end of the pipe is outside of the house , 6 inches off of GND and facing down. Is it just serving the water heater or the whole house?
The common thing today is to have a thermal expansion tank. But if you don't have such a tank, you could connect the pressure relief pretty much to any pipe in your plumbing, and it would release the excess pressure. In other words, it does not have to be right at the WH. Could be in the laundry room, for example.