This project demonstrates how to script "sticky" Subtitles - meaning, this technuque uses a GPRM to keep track of Subtitle status/value...

This project uses Scripts and SPRMs to set and track the Subtitle Stream that the Track will play.  The project uses scripts to set and retain the selected Subtitle Stream number in a GPRM, so that the Stream selection will persist during playback from Track to Track. ALL Tracks MUST contain the same number of Subtitle Streams, or the stream selection will fail to "stick"..

Specifically - If you set the Subtitle Stream to something OTHER than "0" (the first one), and play ANY Track or Slideshow that contains fewer Subtitle Streams than the selection, the stream selection will revert to an uncertain number (usually Stream zero).

The way to prevent this is to place dummy subtitle streams into any Track or Slideshow that contains unused stream areas; So if you have a Movie with 2 subtitles (0, 1), and a Special Features with only stream 0, you MUST put a dummy (textless) subtitle stream into the Special Feature sub stream 1 (the second one), or playing it will cause the Subtitle Selection to revert down to stream 0.