Null Flavor

A value may be null, but null why. Different Null Flavors convey entirely different meanings of Null. Is the data missing because it wasn't provided, or was deleted, or was invalid and rejected, or was just never requested in the first place because the data couldn't have had any relevance.

http://dicom.nema.org/medical/dicom/2017c/output/chtml/part20/sect_5.3.2.html