It’s usually flat on the rad and pressure release on expansion tank.
That’s my experience where that twin cap is used. My Plus8 had them.
The expansion tank is there to deal with coolant expansion as it heats up. It catches the increased volume and returns it to the system as it cools. Excess coolant and pressure build up opens the cap on the expansion tank to let it escape via a drain pipe. When excess pressure is released the cap closes up again. Basically an excess pressure release valve.
