DICOM PS3.4 2018e - Service Class Specifications
C.2.2.2.3 Universal Matching

If the value specified for a Key Attribute in a request is zero length, then all entities shall match this Attribute. An Attribute that contains a Universal Match specification in a C-FIND request provides a mechanism to request the selected Attribute value be returned in corresponding C-FIND responses.

DICOM PS3.4 2018e - Service Class Specifications