The linked article below is from 2010 but I assume things are still pretty similar. It sounds like there are temporary contracts which are usually for the first couple of weeks of the season and then full contracts which are for the duration of the summer. The temporary contract players appear to be fill-ins until players from the College World Series begin to free up for playing and also possibly players that were trying out for Team USA. Here is another article from 2019 that is somewhat informative as well: https://bournebraves.org/news/temporary-players-taking-ad...