P1393 Maintenance Rules: "Date - Projected From" and Through Date setting of 2:

How or what is the Through Date arrived at when the P1393, "Date - Projected From" field is changed to a "2".

We are fairly familiar with the PM Maint. Rules and use of the Projected Start Date Method, Sampling Days and Date - Projected From. We currently use the value of "1" in the "Date - Projected From" field.

All the KB documents and other information as well as examples use the Date Projected From with a value of "1", which is the last competed date in the F1207.

There is another value of "2" which can be used which is described as "Through Date".  What exactly is this date and where is it derived from?  R12807 Processing option? A system Date?