Permissions are related to the fact that Umbraco can host nested content structures, meaning hierarchies of content items of different document types. Permissions determine which types of structures are allowed.
For example, imagine this structure of content items belonging to these Document Types:
As the owner of the web site, I would like to prevent that editors would be able to insert Team Member content items under Products or a Product content item under Team.
So, now, a Permission defines mainly two things:
The appropriate Permissions for the simplified structure would be:
In other situations, you may want to allow recursive nesting of the Document Types in the content structure. For instance you might want to allow this structure of Document Types:
The allowed Document Types for the Content Type Document Type would be Comment and Content Page.