Use project roles to manage what mail types, document types, and document statuses an organization has access to.
If you’re a Project Administrator you can do the following:
To prevent organizations from overriding the mail and document types they have access to, you should lock them.
Setting a default role is useful because it's automatically assigned to any new Orgs that are added or invited to to the project. So whatever mail and doc types, and statuses, are added to it are available to a newly added org without you needing to do anything.
So if you're managing roles, it's a useful safety net to make sure new orgs don't get access to more doc and mail types, for example, than they need. If you’re going to use a default role, it’s a good idea to create a role with only one mail and doc type, and use that as a default. You can then assign the org the correct role when you're ready.