This is the ValueSet that contains all the codes in Modality (http://dicom.nema.org/medical/dicom/current/output/chtml/part16/sect_CID_33.html) and Modality (http://dicom.nema.org/medical/dicom/current/output/chtml/part16/sect_CID_33.html). E.g. what you have to deal with if you get resources containing codes in either of them