The complete permissions model is here showing the functionality availble to users with particular licence and permission mix:
These permissions will by default apply to every document and file in the project. If required the permisisons associated with a Group can be altered to give higher or lower permissions to a particular document(s) in the project. This is done a document at a time with Document level Permissions and can also be reviewed and altered for the entire project.
Additional rights are granted by an Administrator Account and these work in conjunction with the user permissions level to determine what each user can do in the system. Some of these Admin rights are delegated to a Modify user on the project in the 'PM as admin' mode.