When Captain Stewart arrives in Walmington to recruit the platoon, Captain Mainwaring misinterprets his hesitancy in describing what exactly they are required for to mean 'special duties' – Stewart allows him to believe this to get his co-operation.
Captain Mainwaring misinterprets Captain Stewart's hesitation about describing the nature of their assignment when he arrives in Walmington to recruit the platoon, believing it means "special duties."  Stewart plays along with this misinterpretation to get Mainwaring on board.