Let's say priority 1 contains the thrusters and some other things which run within the plant output (life support, FSD, etc.)
Priority 5 has the SRV.
For simplicity we'll say no other modules are turned on.
When in normal flight, there isn't enough power to run the SRV bay, so as it's on priority 5, it automatically powers down and the rest of your equipment keeps running.
When you land, the thrusters power down. Even the smallest thrusters use more power than the largest SRV bay, so there's now enough power to run the SRV bay, and it powers up.
When you dismiss the ship, the thrusters power up again. Now there's not enough power to run both "1" and "5" together, so 5 is shut down. There is now enough power to run the thrusters.
Recall the ship, it lands, the thrusters power down, and you can now run the SRV bay again.
From the SRV, you can get the ship to take off and land near you?! Cool, if that's the case.