I guess that I visualize starports as a cross between sea ports and airports, as well as what I encountered in the Solomon Islands.
A Class A starport has the ability to build new civilian ships, and may have a facility capable of building warships, It is able to do major repairs on all civilian merchant ships and some repairs on military ships, unless it has a warship construction facility co-located with it. Typically, it will have an orbital port in geo-synchronous orbit over it to handle very large and non-streamlined ships. It will have the ability to handle and process large quantities of cargo and passengers without major slowdowns.
A Class B starport has the ability to build space ships and do major repairs on civilian merchant ships. It may have a small military building facility for system defense boats, in which case it can perform moderate maintenance and repair of military ships. It can handle and process large quantities of cargo, but may not have an orbital port as well.
A Class C starport has the ability to do major repairs on small, less than 1000 ton, civilian ships as long as they are streamlined and can land. It does not have the capacity to repair warships but can handle replenishment of life support systems and consumable supplies. It has some cargo warehousing and handling capacity, and one or two passenger hotels on the starport area. It will not have an orbital port.
A Class D starport has no repair capability, but is capable of furnishing replenishment of life support and consumable supplies. It will have cargo receiving and shipping personnel, but no warehousing or cargo handling on the port area. Passengers have a small terminal for waiting.
A Class E starport has customs, immigration, and medical clearance personnel representing the local government (all other ports will have this as well). It will have a designated landing area, probably a concrete pad, and at least one cargo broker will be available. No significant supplies can be furnished. Cargo and passengers are hit and miss.
A Class X landing site means that you are looking for the best place to set down, hopefully near something worth while.
The last three, D/E/X, approximate what you would encounter in the Solomon Islands. At Honiara on Guadalcanal, you have a dock for the port area, no repair capability whatsoever for anything more than an outboard, but you can get supplies and fuel (expensive fuel). The airport, Henderson Field, has a small terminal building, some refueling capability (a couple of fuel tankers), and is basically still the World War 2 bomber strip, hard packed coral, no concrete.
Gizo, further up the chain in the Solomons, in the New Georgia group, has a dock, with a hotel about 100 yards away. The customs, immigration, and medical clearance officer is one in the same person, and hopefully will be there when he is needed. No ship diesel is available, and the small air strip is located on an adjacent island in the harbor, with refueling done via 55-gallon drums and a hand pump. (That really freaked out a couple of ladies in the National Geographic group. Personally, I thought that is was pretty cool,) Cargo, passengers, not really, except for empty 55-gallon drums, lots of those, and the odd outboard to be taken to Honiara for repair. Did have some really nice bananas, which unfortunately lasted only 24 hours after being picked, really good rock lobster, and lots of coconuts. Plenty of fresh fruit for the crew, but not really anywhere near enough for an export trade.
And then there was Ringgi Cove. Big enough to put a small ship in, and that was it. If no native available in the area with dugout canoe or launch, break out the ship's boat and head to shore. No navigation lights at night, and it gets really dark there at night, without civilization light pollution. A classic Class X starport.
Edit Note: A world may have more than one starport, but only one Class A. If it is large and has multiple continents, you might have some additional Class C and Class D starports around.
