Slope, y-intercept and incline angle of the line through two points.
That happens when both points share the same x-coordinate, making the line vertical. Slope is Δy/Δx, and dividing by Δx = 0 is undefined — the calculator detects this and reports the line as vertical instead of showing a slope.
It's the line's angle of incline in degrees, found from arctan(slope). A positive angle means the line rises left to right; a negative angle means it falls. It ranges from just under +90° to just under −90°.
Once the slope (m) is known, the y-intercept (b) is solved from the line equation using either point: b = y1 − m×x1. That's why the y-intercept only appears when a slope could be computed (not for vertical lines).