DICOM Private Elements
Why Private Data Element Implementations may require communication of information that cannot be contained in Standard Data Elements. Private Data Elements are intended to be used to contain such information.
Private Data Elements have the same structure as Standard Data Elements (i.e., Group, Element, VR, Length and Value). To distinguish private data element from Standard ones, the group number used in the Element Tag of Private Data Elements MUST be an odd number.
Private SOP class
A Private SOP Class is an object constructed and transmitted in the same basic way as an official DICOM SOP Class but with the contents defined by the creator of the private SOP class, rather than by DICOM. Typically a piece of equipment which uses a private SOP class will try to negotiate both a Private SOP class and an official DICOM SOP Class for the images they send to a PACS, on the basis that only a PACS from the same manufacturer will accept the private one, with everyone else accepting only the DICOM one.