The group content system is an add on to the core system of Drupal. By default roles and permissions apply to content types globally, don't restrict access by menu or section of a site, or fields. We use groups and a system called Organic Groups as a combination of modules that allows you to segment off content for specific groups or departments to have control over. When it's more of a sporadic approach to content that someone may need to edit, but not housed within a "section" of the site, sometimes we'll use taxonomy access control instead. That is possible but rare that your site would use this.
Since the system by default isn't built in a way to handle controlling content by specific sections of a navigation or site area, groups gives us that control. By general configuration:
- Groups have content types like the rest of the website
- Groups have group members that have roles and permissions within the group or groups they have been added to, allowing editing or adding of content by group content type
- Have group admins that control the membership and the content
- Groups have their own menu, even if group content is within a main menu, the group can have it's own menu.
In the group system setup, you can define the group type. For our documentation sake, we'll say it's a department section of the website. In the department setup we configure the ability to create pages, within a section of the site, and within specifically the menu the group owns, not allowing for content creation outside of that group's area or URL path. We also configure permissions on who within that groups users roles could edit that content, or perhaps other special options based on the site need.
Site users can be in multiple groups, allowing the ability to manage content within each group or creation of content, but they still wouldn't have global rights to manage a menu or global content.
If a user has a global login, and special roles in terms of editing or creating global content types, AND has group roles and permissions, all the permission systems in the site are cumulative allowing all the permissions. That means if they can add news articles to the global site, and have a role in a group allowing the posting of pages to a group, they will be allowed to do both.