Repository
When you are on the Source edit screen, there is a button to click to show the repository. The repository is the name and address of the place that has possession of the source. If the source is a book, the repository might be a library or an individual. Basically you want to identify where someone could go to find the source that you used. If the repository is a library, you can also enter the call number with the source. The call number is what identifies the source at that repository.
If you link a repository to a source and later want to delete that link to the repository, you may do so while on the Source screen. Pick from the top menu, File, Delete link to repository.