In a low wing set-up, the fuel gets "sucked" up by the fuel pump(s). Hence no "both" setting - try to suck up from an empty glass and a full glass simultaneously through a pair of straws, and you will only get air.
dH Chipmunk - low wing, fuel selector offers only 'on' or 'off' selection.
Whatever you do, if you change tanks before t/o give the engine time to stop (if it's going to) while you are still on the ground. Chap a couple of years ago who I knew didn't, and died following EFATO.