DICOM PS3.5 2019a - Data Structures and Encoding

7.4.5 Type 3 Optional Data Elements

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

DICOM PS3.5 2019a - Data Structures and Encoding