The buffer stream-time, also known as the position in the stream, is calculated from the buffer timestamps and the preceeding SEGMENT event. It represents the time inside the media as a value between 0 and the total duration of the media.
The stream-time is used in:
Report the current position in the stream with the POSITION query.
The position used in the seek events and queries.
The position used to synchronize controlled values.
The stream-time is never used to synchronize streams, this is only done with the running-time.