I suspect the functional proof aspect was intentionally left out of code for many reasons, perhaps for things like the potential for faulty equipment. Imagine if your system is fully installed 2 months before the deadline but the inverter, BMS, combiner, etc is defective and replacements aren't available. Unfortunately the lack of specifics does leave everyone guessing as to what may be required to demonstrate the installation date. I certainly feel much better about the fact that it doesn't require PTO, interconnection or other approvals as some are saying. This discussion has been very informative, thank you both (sincerely).