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In this article Although sites that are built on Windows SharePoint Services often have additional default SharePoint groups, Windows SharePoint Services 3.0 includes five permission levels (permission level: A set of permissions that can be granted to users or SharePoint groups on an entity such as a site, library, list, folder, item, or document.) by default.
One of the largest causes for complaints in previous versions of SharePoint was the lack of Securable Objects (SO) that existed only allowing end-users the option of securing items at the library level.
Recently, I was involved in a support request where I had to find out what SPBasePermissions are assigned behind permission level in SharePoint using SharePoint OM code.