The details are that there are no details. Who ever says it needs a TSO needs to provide the documentation saying that it must be TSO'ed. Easy to do for something like a Transponder. But mostly there isn't anything requiring a TSO for a comm radio. Maybe for 135 or 121 operations, I haven't looked.
Lots of equipment is commonly installed in certified aircraft that never had a TSO.
Somebody please let me know if they can find the TSO for a G-meter, clock, or Vacuum Guage.
Even if a TSO exists, there still needs to be a requirement that the instrument being installed needs to meet the TSO. like FAR 91.215.
It is possible that a manufacturer might require certain equipment installed in the aircraft to meet the TSO, but again documentation should be provided before saying the equipment you want to install must meet a TSO.
Pilots, Mechanics and even the FAA frequently like to quote rules that don't exist.
two I have heard from FAA inspectors...
Fuel caps on a 172 must be painted red?
Passengers can not be carried when towing a glider?
Brian