If I understood you correcly ... both the ETC rack and the fuel tank were the option. So these could be attached or not depending on the mission type. Eg. the JV44 Doras were defended the airspace over the Me 262 airfield so these didn't need the additional fuel tanks and racks for them. Also that's possible there couldn't be anybody who could take care of the fuel tanks and racks storaging them separately at an airfield that could be changed quickly additionally. So the racks could stay attached although not being in use.