I was watching Raiders of the Lost Ark with some friends when they did one of those old style maps with the dotted line marking the passage of a plane Indy was on when I said, "Navigation was pretty hard back then for pilots." One of my friends looked a little bewildered and said, "How so compared to today?" Not a tech minded person. I explained, "Well they didn't have global positioning satellites or even tower lights. You could get lost flying across the USA. That's why they got crop duster planes to fly along the regular routes spraying loads of paint to mark them for the pilots." Well my friend was buying this completely until I overplayed my hand and said, "Yeah, that's where they got the idea for that dotted line on the map scene they used in the old movies." I got smacked by her for that.
Yes. I can be a bastard. Have you read my tables on stuff that can go wrong? On to the post.
There was a time, in Classic Traveller, where the referee was instructed to roll for commerce and communication lines. You basically rolled for each world and based on the starport you could have a regular line of communication a/o shipping (up to four parsecs away for a Class A 'port). As the game evolved this little table was done away with. You can still find it in the Lost Rules Supplement long with other good stuff.
Nowadays referees merely place communication lines as they deem appropriate. It works I guess but it ignores a source of information to determine subsector politics.
I'm pretty sure it'd be de clase to reprint the whole table here and copyright infringing. Go buy the CDs. But the gist of it is Class A-B ports can have commerce lines linking them up to 4 parsecs away. B-C type starports up to 3 parsecs and D or lower 1 or rarely 2.
Usually in CT civilian and low priority government ships have J-1 or J-2. Military and express shipping (government and civilian) average about 3-4 and the really important stuff and bleeding edge warships move at J-5 or J-6.
A commerce line of one or two parsecs indicates a healthy trade between worlds but nothing shocking. Fat and Far traders and Scout/Couriers running mail. You'd expect an A port to have several lines to adjacent worlds. An A port in isolation might indicate a world being censured or with an embargo or even blockade. A good starport with no adjacent links but a three or four parsec link to a single world might be economically dominated (or the dominator) and there may be several monopolies being protected there. A three or four parsec link indicates something is going on. Perhaps two worlds see fit to coordinate their military actions or one world is supporting a research base or other enterprise needing constant updates on progress or priority shipments.
A lack of commercial lines might also indicate the area is rife with pirates or privateers. Believe it or not most free traders who are npcs prefer to avoid danger. The same applies to active warfare. You didn't think mercenaries were only ground pounders, did you. Some worlds with no commercial lines might be xenophobic or have repressive societies that wish to limit offworld contamination.
So in summary when you're trying to make sense of the size 1 worlds with standard atnospheres and the desert hell holes with 20 billion people give some thought to the commerce lines and any anomalies that might suggest adventures.