On many platforms and file systems an entity requires appropriate access rights or permissions in order to access objects in a file system. The access rights are generally performed by checking the identity of the entity. For example, on implementations that use Access Control Lists (ACLs) to enforce privilege separation then a file in the file system may have an associated ACL that determines the access rights of identities specified in the ACL.

A UserPrincipal object is an abstract representation of an identity. It has a name that is typically the username or account name that it represents. User principal objects may be obtained using a UserPrincipalLookupService, or returned by FileAttributeView implementations that provide access to identity related attributes. For example, the AclFileAttributeView and PosixFileAttributeView provide access to a file's owner.