Usually you need special flooring with tie-downs and rollers to facilitate moving the load around, which helps maintain an appropriate CG (Center of Gravity).

Usually you need special flooring with tie-downs and rollers to facilitate moving the load around, which helps maintain an appropriate CG (Center of Gravity). Purpose-built freighter models also remove things that aren't necessary and just add weight - such as windows (packages don't need a view). This allows the carrier to transport more cargo in the same flight.