Timing Information MPEG-2 TS

This question is specific to MPEG-2 TS and this:
*         startTime is set to the time in the media resource timeline when the first private data packet in the DataCue was received.
http://www.w3.org/community/inbandtracks/wiki/Main_Page#MPEG-2_TS
In current proposal, textTracks with cues of type metadata are expected to have startTime's populated .For PES packets the PTS may be available (although not guaranteed) however for private payloads the PTS might only be available from, say, a video frame. However, the transport stream may have many video/audio streams that may not be directly associated with this metadata track. So the question is, which PTS would be used to timestamp this cue data? Or is another clock expected to be used (which BTW might have similar issue)?

Regards
Graham Clift
Sony Corporation