segmentType attribute of pbcoreDescription: What for?

[jackbrighton]

There must be a reason segmentType was included as an optional attribute of pbcoreDescription, but I'm not getting it. I can see wanting to annotate a media asset based on startTime and endTime, but that wouldn't require the attributes segmentType, segmentTypeSource, segmentTypeRef, segmentTypeVersion, and segmentTypeAnnotation. It's not clear from the documentation what a segment even is, and how it's different from a Part, or even just using a pbcoreDescription element with time annotation attributes.

Is there a use for the segment attribs that I'm not getting? If so, the documentation should explain this and provide an example.