What the heck is a Scheduled Cargo carrier?
The ship has a preset route and it just follows that route regardless (like a train on rails, effectively). The timing won't be perfect (because jump durations vary from 150-175 hours per jump and not all jump shadows are the same size, so maneuver times can vary too) ... but there's a kind of "mark you calendar" effect going on where parties that want to ship to the "next destination" on the itinerary will be able to plan accordingly.
Contrast this with a Tramp Cargo carrier.
There is no preset route.
Where the ship goes next is entirely determined by where is the most profitable place to go from HERE
at this time while the ship is here conducting business.
If there's a speculative cargo that they can buy cheap here that has high value on a Non-industrial world, they may divert there to sell that speculative cargo.
If there aren't enough cargo and passengers wanting to go to Destination A, the ship may switch to Destination B instead that more cargo and passengers are wanting to go to instead.
The Scheduled Cargo carrier is following a preset route.
The Tramp Cargo carrier is going wherever the economic winds are blowing "best" right now.
Being chartered (or not) is another layer on top of that.
A Scheduled Cargo carrier could be chartered to run an entire preset route.
That way, they aren't subject to the ups and downs of the markets on each of the worlds they visit. They just GO and drop off/pick up whatever is there and get paid regardless of however much there is to load up and drop off. They're basically the "mailman" making rounds and delivering on a preset route ... but they're doing it under contract to Someone Else™ who wants to have the ship making those rounds in that order so as to move those passengers and goods (however much there is of both from each stop).
A Tramp Cargo carrier can fly in, announce that they're "available for charter" to wherever the contracting party wants the ship to go to on a First Come First Serve basis. Whoever ponies up to pay for the charter first hires the ship ... and then it's the contractor's responsibility to fill the manifest going to that destination that the contractor wanted to get to. If there are no takers, then the Tramp Cargo carrier can try to drum up some individual business to fill their cargo hold and then use that to decide where they want to go to next. Once the decision on next destination is made, they announce and open availability to passengers wanting to travel to the announced destination.
Point being that charters can be a long term thing (we're hiring your ship for the season/year) or it can be an ad hoc one off thing (we're hiring your ship to go to a single destination). Subsidized or not makes not difference for this. Subsidized ships can operate on a preset schedule or they can operate as tramps.
It really just depends ... although multi-destination charters tend to come with preset routes chosen by the contracting party (for whatever reasons suit them best).
The trick is knowing where the break even point of cost vs revenue is under different working conditions with your ship in order to ensure that you earn more profits than losses over time. Charters may be "guaranteed money" ... but if you aren't buying up any of the cargo space from your contractors (net Cr100 per ton to them) aboard your own ship under charters, you'll have no free cargo space of your own to speculate cargo with yourself. So it can sometimes come down to how much of a "sure thing" versus a "gambling thing" you want to engage in when it comes to offering charters.
Also consider that big ships can't necessarily load and unload in "zero time" per se. Once ships reach a certain size, it can take them long enough to unload and load to affect the tempo of their Destinations Per Year, slowing them down enough that on an annualized basis they aren't as profit efficient as a smaller/faster/nimbler ship with quicker turn around times. You can wind up with all kinds of edge cases at different technology levels, map considerations (which systems are where), population and trade codes ... it can all get REALLY complex in a hurry!
And mail is "dedicated cargo space" that can't be used for other purposes.
