What are loc.first and loc.second in outputTrackParameters?

[wdconinc]

There are two locations in outputTrackParameters: loc.first and loc.second. Are they start point and end point of this track in the tracking system? They should not be hits because a track should not only include two hits.

[wdconinc]

Track reconstruction with Acts is based on the use of local coordinates on surfaces, which are always a pair. Track finding/fitting returns the local coordinates on a PerigeeSurface where loc encodes the information of d (first) and z (second), where d is often called the distance of closest approach to the z axis and z the longitudinal position of the closest approach.