This is set to 1 when right when you leave Burgh de Rott or Paterdomus. It's set to 0 when you leave Temple Trekking via a teleport or by "Escape"-ing the Trek, and also set to zero when you go to the next event. If no event is encountered on the Trek, this value stays at 1. This can also be kept at 1 by logging out mid trek.
The purpose is unknown.