But I noticed that it is only Starfleet officers using credits-- My guess is that Starfleet officers are given an account where they earn credits to use with other cultures that use money. For things like shore leave, purchasing special items, conventions, alien space stations. It's a must. Jake on the other hand being a civilian, doesn't have any money--not even Federation credits. As long as he used the Starfleet replicators, he's fine. Otherwise, he was pretty much helpless to do anything that required money. It's times like this that I wish one of the Trek shows was still airing on TV because it's so much fun when you notice certain things that stick out. You just have to appreciate how odd some of this looks in light of what the characters sometimes claim. It's like Humans apparently go through all the motions of a work/commerce without actually doing it. Jake sells his stories to the Fed News Service--but they pay him nothing because Earth has no currency system. He uses the words, describes the process, but he was not paid anything. So then you have to wonder--in the 24th century, where all needs and wants are met easily...no poverty, no need for money..... ...if there are maids or janitors--and there is no money or compensation to be earned and at times you have to clean up disgusting things, or put up with an obnoxious boss who talks down to you--why would they do it? It's just interesting to watch.