Every repository must have a type associated to it. For example, if you create a repository that will link to a GitHub repository, then the repository type must be set to Git.
Once a repository has been given a type, more repository type specific properties will be added to the repository dialog via the tabs at the top of the dialog. When creating a repository, if you change the repository type then the repository properties will also change to reflect the new type.
Note that you can only change the repository type when creating a new repository. You cannot change the repository type when you are editing an existing repository.