Checking Documents Out From the Repository

To check project, model, or file documents out from the repository, you must have read, write, or full permission on the document or (for a model), one of its packages. Documents for which you have only a List permission or no permission at all cannot be checked out.

You will want to check a document out from the repository either to add it to your local workspace, or to update your local version with changes made by other users.