The Vector Ruler is a tool added by Psi. It will measure the position vectors of any block by using one block as the origin point. When held, it will remember the position vector of the last block it was right-clicked on. Shift right-clicking with it on a block will set that block as the origin point. Right-clicking with it on a block will display the relative position of that block to the origin (e.g., right-clicking on a block 2 blocks up from the origin will give a vector of [0, 2, 0]). It is intended to be used with
Selector: Ruler Vector.