I just found discovered today that if the mission "requires" smokejumpers you have to send them by plane. I had a mission today that required "20 Smokejumpers". I had Brush Trucks in that area with all folks trained as Hot Shots and Smokejumpers, so I sent 10 brush trucks more than the 5 required on mission (meaning there would be 20 smokejumpers there. But it still would not finish until I sent a plane with smoke jumpers.
yeah - smoke jumpers - jump (parachute) into some areas that are inaccessible to land crews ( similar to SWAT only counted in SWAT vehicles)