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.

Most Private SOP classes are very similar to official DICOM classes, and many contain pixel data and pixel descriptive data (0028,xxxx) in standard elements, allowing them to be displayed just like official DICOM SOP classes, but some have totally different uses, such as being used as a demographic update mechanism, and in general it is important not to accept SOP classes that you do not know how to handle - see the article Avoiding accepting private SOP classes for more information and example code.