Starports can be located anywhere. The MAIN starport is going to be located where the most shipping occurs for that system. I doubt many primary starports are going to be located around a gas giant due to the 100D rule. Usually the most shipping will occur around the most populated planet, but not always.
Where do you think your downport should be located at?
(Near a city/living area or far away)
(On land, on an island, on water)
(Close to industry or isolated)
Downports are usually depicted next to population centers. Though that's not a hard/fast rule. The port is always going to have its accompanying 'star town', but beyond that the planet may be hostile or indifferent towards the Imperium and the port itself may be located away from population area. I would think most are going to be located where its cheap to build the downport. While you could build it over water, its going to cost more, but sometimes that's the only choice you have.
Do you need a Starport if you have a downport?
Do you need a downport if you have a starport?
Class A & B starports are always going to have an orbital element to them to support larger, non-streamlined shipping. Not to mention acting as pure trans-shipping points. You may actually have multiple orbital stations orbiting a planet located near the edge of the 100D limit (like above and below the plane of the ecliptic) to service purely transhippers and other ships who are just passing through to pick up/drop off cargo and passengers.
Smaller starports are going to be "iff" for the orbital component. Lower tech and small starports (D,E,X) are most likely going to be limited to some sort of traffic satellite in a stationary orbit, where all ships need to land or send down shuttles to do business and drop off people/cargo. If they do have an orbital component, you'd most like see a very small facility.
If you have a downport, what is the maximum tonnage of a ship that would be allowed to land?
Or is the ability to land safely the only requirement?
Typically you won't see very large ships coming in for landings. They are just too unwieldy in an atmosphere. I'd say you probably won't see anything more than 10,000 tons on the ground, but most likely you'll just see things in the 5,000 or lower tonnage range.
One thing to keep in mind is that a downport needs the facilities and space to land something big. So keeping a 10,000 tone pad available for larger ships is going to cut into their ability to service the smaller freighters and free traders. But commercial entities who stop there regularly probably will have their own facilities to load and service their own ships.
If you have a downport, are all ships required to land there or can they land anywhere they want on the planet?
If they can land anywhere on the planet (downport or just a starport) are they required to inform a Central control or just go where they want to go?
That's going to depend on local laws. On a balkanized planet you are going to see multiple starports, but only one 'main' starport under imperial authority.
Most localities are probably going to restrict starships to specific areas/ports. Smaller ships such as shuttles and cutters would have more flexibility to land and pick up/drop off cargo, but I would bet a higher law level planet is going to centralize it all to keep their eye on everything.
