IODs and SOP Classes define Type 2 Data Elements that shall be included and are mandatory Data Elements. However, it is permissible that if a Value for a Type 2 element is unknown it can be encoded with zero Value Length and no Value. If the Value is known the Value Field shall contain that value as defined by the elements VR and VM as specified in PS 3.6. These Data Elements shall be included in the Data Set and their absence is a protocol violation.
Note: The intent of Type 2 Data Elements is to allow a zero length to be conveyed when the operator or application does not know its value or has a specific reason for not specifying its value. It is the intent that the device should support these Data Elements.