x1 is the spacial interval/geodesic segment (or line segment/ray/line) from location/point x2 to location/point x3 in geometry/frame of reference x4.
Like "temci", but for space. "nicfa" yields a vector which encodes similar information (when projected into space). x4 can be used, for example, in order to determine whether x1 lies across the Earth's surface or cuts through the Earth (like a tunnel) for two distinct city centers. Like "temci", it is essentially just "sirji", but specifically designed in order to pertain to a given measurement-dimension (namely, in this case, space).