Yes it's track miles.
Many STARs bring your ac closer than 15Nm from the airport.
Auto activation of the app when you have 40 track miles or more to go would not make sense.
As far as I know, the "Decel waypoint" is only here to assure a continuous deceleration so that you reach:
- 1000ft AGL at Vapp in landing conf (for precision Approaches)
- the FAF/FAP at Vapp in landing conf (for NPAs) Provided you inserted Vapp on it
- a speed constraint which is below green dot somewhere in the STAR