Compressor/air handeler compatability
Coils for 14 SEER plus are very large, and often take either an X13 or VS blower motor to get it. Your made for 12 SEER coil will have fewer tubes in it and fewer fins in it, which will lower heat transfer. Probably won't have the right size meering device either, unless it has a TXV on it.
Its measure by testing it at 5 different outdoor temps to see if it reaches its rated capacity within 10 minutes, and by checking the amount of energy it used in that 10 minutes.
2 test were conducted by 2 different companies pairing 10 SEER indoor coils with 13 SEER condensers. Some got only 8 SEER, a few got a mid 11 SEER. But had to be over charged to do it.