Good advice noted. I had thought this was a general question, but it is
now clear that defrost implementations do vary somewhat.

Yes, indeed, it was a Trane heat pump.
But no info with it. I rarely see info with units.

Honestly, I should have figured it out from the schematic.
I jumped to some bad conclusions based on my limited knowledge.
Going forth, I will be better armed.

Thanks again to everyone for the help.