A freighting-line base was a type of colony founded with the purpose of being a waypoint on a freighter route.
The planet Deneva was originally colonized over a century prior to 2267 for the purpose of being a freighter line base. Spock noted this while the USS Enterprise visited Deneva that year. Montgomery Scott added that regular trips were made to transport freight from the asteroid belt to miners and bringing back cargo. He himself had made the run "a couple of times" as an engineering advisor. (TOS: "Operation -- Annihilate!")