SCR is indeed the timebase of the stream for program streams (VOB is a program stream). The equivalent for transport streams is PCR. You can use the Log Timestamps function of DGIndex to see that the SCR resets along with the PTS.
A decoder should align a local clock with the stream's SCR. Then, an access unit is displayed when it's PTS (possibly interpolated) matches the local clock.
My point about the wrap-around is that it *can* occur. Also, in broadcast environments, the timebase and associated timestamps can all shift when a new stream is sent (e.g., new program, cut to commercial, etc.) This is called a discontinuity and decoders must not be designed to assume that there are no discontinuities.
Using the starting and ending PTS to determine a stream duration is a heuristic that is foiled by any discontinuities.
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