@jmk yes the Seasonal energy efficiency ratio or SEER a high number is better.
The system won’t let me comment must be because I am new on this page of the SE .
There are similar questions asked on DIY home improvement.
Basically the outside unit is the compressor and condenser coil.
The inside unit could be a very basic coil and orifice and your system would be a low seer value system.
By increasing the coil inside the evaporator to have more fins so the heat transfer is better this brings up the efficiency.
To gain additional efficiency we ad active controls called txv (thermal expansion valves) these valves regulate the amount of liquid that is allowed into the evaporator by adjusting the valves based on the temperature.
By metering the refrigerant slowly or just what is needed the compressor cycles less often increasing the efficiency.
The inside coil size and type with a TXV and possibly a multi speed fan are all that can be done there to increase the efficiency of the system.
To increase the overall system beyond that of txv and evaporator coil the outside unit can have a better condenser coil.
(Rare but I did this 1 time when dogs peeing on the outside coil caused it to fail)
The other thing that can be done outside is an inverter driven compressor. I have not upgraded to an inverter.
Usually the outside unit is chosen and we match to it or slightly better.
The cost although off topic can be 2x going from a 14 seer system to a 21 seer system.