passenger airline pilot union scope usually forbid airplanes that carry in excess of a certain # of pax.
same scope issues at a cargo operator, however, the scope will usually forbid the company using an operator/aircraft that can contain a ULD, igloo, whatever you want to call it.
brown used to load the E-120 through the rear cargo door, box by box. took'em 2-3 hours a day to load the entire cargo hold. also, if your haulin' freight under 135 rules, your limited to 7500lbs.
FG