TH, TR and TH/TR
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)