In Forms authentication mode, whether to use or as the underlying mechanism to identify a user.
Assembly: Microsoft.SharePoint (in Microsoft.SharePoint.dll)
ASP.NET MembershipUser.ProviderUserKey is a unique ID separate from the user name. if you set the farm at mode, then SharePoint Foundation will use instead of MembershipUser.UserName to identify user. Setting this mode before adding any membership provider users into the farm is recommended, because changing the mode does not migrate existing user entries in SharePoint Foundation database. Also, there is a performance consideration. Calling may cause extra database traffic to the membership provider database, depending on how the membership provider is implemented.