Great question, Jamie. So you need to check with each publication, but in general it's understood that they are buying "first publishing rights" (which means you haven't published it anywhere else even on Medium). Also, they usually have an exclusivity period--often a month or two, but it varies. After the exclusivity period is up, you can republish it wherever you like. You can also sell reprints of stories that you have published on Medium (second publishing rights) but it will be for less money and there are less publications keen to buy reprints. I would be fairly reluctant to sell all rights for a story to a publication, but I've only seen that a few times and I mean, if they paid enough I might!