I've left it open-ended, so you can set the fares to every X unit costs Y. When you create a schedule, you have the option to select a flight type, so you can select Cargo there. It doesn't change the units or anything, but maybe if it's a cargo flight, it can change the terms used to be more cargo-specific for those flights. But that's sort of what I was envisioning, with the open-ended nature of how fares can be set on a subfleet... which would allow you to mix a subfleet that's dedicated for pax and another for cargo... if that makes sense.
Aside from unit cost, what other freight-spec