I just FIND !!! for anybody to have the same problem , just go on customize position and click underground fuel
That's only required if you don't want to use the default fuel vehicle assignment strategy, which is to always use a Tanker if the parking is on the Apron and always use an Hydrant, if the parking is on a Gate.
The underground fuel flag is there to allow you to override this logic, and use an Hydrant on Aprons, or a Tanker on a Gate ( not all airports have underground fuel services )