Subcooling.
How far below its boiling point the liquid refrigerant is. The charge measurement on a TXV system.
Subcooling is how many degrees below its condensing temperature the liquid refrigerant is as it leaves the condenser.
It is measured by taking the pressure on the liquid line, converting that to the temperature at which the refrigerant would condense, and subtracting the actual measured line temperature. The difference is the subcooling.
What it tells you
On a system with a TXV, subcooling is the primary charge measurement.
Low subcooling usually means not enough refrigerant in the system, which means a leak, because a sealed system does not consume refrigerant.
High subcooling usually means too much refrigerant, or a restriction in the liquid line such as a blocked filter drier, backing liquid up in the condenser.
Overcharge is worth naming because it is a service-induced fault. Somebody adds gas without measuring, and the result is high head pressure and a system that trips on high pressure every afternoon.
Why it is not enough on its own
Subcooling tells you about the high side. Superheat tells you about the low side. Read either one alone and several very different faults look identical.
The classic trap: a TXV stuck mostly closed starves the indoor coil and produces readings that resemble an undercharged system. Add refrigerant on that assumption and you now have a restricted system that is also overcharged, and the next symptom is worse than the first.
Read together, the pair separates them.
What this means for you
If a technician added refrigerant to your system, the invoice should be able to say what the subcooling and superheat were before and after. That is not an unreasonable thing to ask for, and the quote guide covers what else to ask.
If refrigerant was added with no leak search and no readings, that is a purchase rather than a repair, and the gas will leave the same way it left last time.