User Identifier

A user identifier is a 4-byte integer value used to uniquely identify a principal within a site collection. User identifiers are assigned from the same numbering space as site group identifiers and cannot overlap. Certain user identifiers are predefined such as the system account (1073741823), the site collection owner, and the secondary site collection contact (2). Values of -1 and 0 are reserved to indicate invalid or unknown user or site group identifiers.