Straight-line distance between two points, in 2D or 3D.
2D mode finds the straight-line distance between two points using just x and y coordinates. Switching on 3D coordinates adds a z value for each point and includes it in the distance formula, giving the true straight-line distance through 3D space rather than just across a flat plane.
It's the distance between the two points if you ignored the z-coordinate entirely — effectively their horizontal separation. It's always less than or equal to the full 3D distance, since it leaves out the vertical (z) component.
They're the individual coordinate differences that feed into the distance formula before squaring and summing. Seeing them separately helps you check which axis contributes most to the total distance.