Value Representations (VR) are format or Type definitions for DICOM elements (Attributes). The VR for each data element specifies the encoding of the data. For example, for all person names, we need an agreement on whether one should specify first the family name or the first name.
The specific VR is specified for each attribute in the DICOM data dictionary. The individual attributes listed in the data dictionary use tags, which are codes consisting of two hexadecimal components. For example, the patient name has a tag of (0010, 0010).
The data dictionary lists all attributes and VRs. For example, it defines that all person names in a DICOM message (i.e. Patient Name, Referring Physician Name, Ordering Physician Name, etc.) have the VR identified with the two-character PN (Person Name). The format of each of the standard VRs is given, including a list of legal characters and any delimiters that one might use.
Explicit VR requires that the transmitted data will include the VR information as part of the DICOM message, indicated by its two character identifier, along with data and attribute tags for each individual attribute. For example, when receiving the attribute patient name in Explicit VR Transfer Syntax, the PN VR identifies the value representation. It includes: Tag-VR-Length of field-Attribute Contents.