SPRM 6: Current Title PGC Number
In projects created with DVD Studio Pro, the Program Group Chains (PGC) number is 1
for all elements, except stories. Each story within a track is assigned a number, starting
with 2 (the track itself is number 1). This value matches the SPRM 5 value except in the
case where a track, story, or slideshow has not yet been played. In that case, it is set to
0.