You’ll run into these terminals commonly on older standing pilot furnaces. TH/TR is a common connection, but that doesn’t explain much in itself.
We’ll overview which each of these terminals is intended for:
TH – The 24v hot leg from the thermostat on a call for heat (R+W closing on the thermostat) to the gas valve (TH terminal) to open the solenoid to allow gas to flow, assuming the transformer is working and the high limit is closed (allowing for the flow of electricity)
TR – The 24v common/return side of the transformer.
TH/TR – Is not internally wired to the gas valve. Not using this makes no difference to the circuit. You could run your 24v hot from the transformer directly to your NC (normally closed) high limit in the above example and be fine. TH/TR is nothing more than a convenience terminal. Joining these wires with a marette (wire nut or terminal multiplier) does the same thing as this terminal