I can't test it at the moment but that's probably solving the issue people were having with sessions being marked 15min prior their real time.
Now we can to stop talking about indicators and focus on the system
Bourgenot,
The vertical line is actually 15 min before the session start, it is on the candle you make the AC reading.
the line is also taking the AC color of the "decision candle" once the candle has closed if the option is enabled (it is by default) :
line is RED -> decision candle's AC is RED -> sell set up
line is GREEN -> decision candle's AC is GREEN -> buy set up.
If it's not clear, send me a private message, I'll explain in french