Excellent question... let me see if I can find some clarity.
I imagine it has something to do with enhanced presence containers and if this is an endpoint that fully supports that.
http://cvegso.squarespace.com/ucma/2013/3/27/ucma-application-endpoint-which-container-to-publish-presence-data-to
(Also, the end is truncated, it's Container list preferred endpoint").
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