Default permission levels in Windows SharePoint Services 3.0
This permission level contains all permissions. Assigned to the Site name Owners SharePoint group, by default. This permission level cannot be customized or deleted.
Can create lists and document libraries, edit pages and apply themes, borders, and style sheets in the Web site. Not assigned to any SharePoint group, by default.
Can add, edit, and delete items in existing lists and document libraries. Assigned to the Site name Members SharePoint group, by default.
Read-only access to the Web site. Users and SharePoint groups with this permission level can view items and pages, open items, and documents. Assigned to the Site name Visitors SharePoint group, by default.
The Limited Access permission level is designed to be combined with fine-grained permissions to give users access to a specific list, document library, item, or document, without giving them access to the entire site. However, to access a list or library, for example, a user must have permission to open the parent Web site and read shared data such as the theme and navigation bars of the Web site. The Limited Access permission level cannot be customized or deleted.
NOTE You cannot assign this permission level to users or SharePoint groups. Instead, Windows SharePoint Services 3.0 automatically assigns this permission level to users and SharePoint groups when you grant them access to an object on your site that requires that they have access to a higher level object on which they do not have permissions. For example, if you grant users access to an item in a list and they do not have access to the list itself, Windows SharePoint Services 3.0 automatically grants them Limited Access on the list, and also the site, if needed.
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Editor's Note: The above article is a sample lesson from the online course, "Microsoft SharePoint for Non-IT Users"