IODs and SOP Classes define Type 3 Data Elements that are optional Data Elements. Absence of a Type 3 Data Element from a Data Set does not convey any significance and is not a protocol violation. Type 3 Data Elements may also be encoded with zero length and no Value, except for SQ Data Elements, which shall not be present with zero Items. The meaning of a zero length Type 3 Data Element shall be precisely the same as that Data Element being absent from the Data Set.