MorphoSource-specific unique ID identifier that was used to group Media together in the now-defunct 1.0 version of the MorphoSource repository. In MorphoSource 1.0, Media groups were used to collect all media derived from the same Imaging Event. Media groups are no longer used, with media-to-media imaging and processing event relationships now being used instead to track hierarchical relationships between Media. Only present if Media was originally contributed to MorphoSource 1.0. Different from morphosource:ID identifier.