Pretty much the same thing with different labeling. They both operate internally in the same way. The labeling of the terminals goes back to original HVAC labeling. Thermostat labeling is color coded: R (red) = 24V hot, W (white) = call for heat, Y (yellow) = call for cooling, G (green) = call for fan, C ( other than R,W,Y or G, but most commonly black or blue) = 24V common. You get an alternative W designation of TH from the common gas valve terminal designations of TH = call for heat, TH/TR = simply a connection terminal with no internal connection to the gas valve itself which is used for inserting safeties into the circuit before connecting the 24V hot to the TH terminal of the gas valve and TR = 24V common. In the case where the W/TH terminal is labeled IND, it is labeled as such because of the action that it causes to happen ( but only after checking that 24V is not present at at PS, which is now labeled TH) , but the 24V signal at IND is still the call for heat from the heater electronic control board and the 24V signal at TH is still proof that the (air) pressure switch has closed after the inducer has started. Also just for clarification if the fan/blower used to aid combustion is on the outlet side of the combustion chamber it is an induced draft blower and if it is on the inlet side of the combustion chamber it is a combustion air blower. F1 and F2 are the normally open relay contacts for one leg of the induced draft/combustion air blower. In older natural draft furnaces the call for heat from the thermostat W went either to TH/TR terminal of the gas valve and through the safeties to the TH terminal of the gas valve or through the safeties connected on the furnace control board to the TH terminal on the gas valve.