It is filled by weight (refill bottle is placed on a scale: not from a can from the auto parts store).
By law the system has to be evacuated (sucked dry and coolant recovered to be recycled), a dye installed then the system is pressurized with compressed air and checked for leaks. If no leaks are found only then, after the system is vacuumed dry can it be refilling with new refrigerant.
That is why it cost so much to take it an A/C shop for repair.
Because auto parts stores sold cans of Freon to end users is why I have been running my A/C at home for the past three weeks. Running 11 to 14 degree above our average temperature of 72 every day this month. Can't wait for summer to see what that brings our way.