Refrigerant travels in a sealed loop between the indoor and outdoor units. It is not consumed like fuel, so a system that is short has lost it through a leak: a flare joint, a cracked braze, a corroded coil or a pipe that has rubbed through. Adding gas without finding that leak means paying for the same refrigerant again, and the system runs short of capacity in between.
So our regas starts with the leak. The technician checks the joints at both units, the coils and the line set, and confirms the leak before recharging. If the leak is in a coil that is not worth repairing, you are told before anything is spent.