This is simply a block to help set the height of the X axis.
We normally print 2 so you can put one at each size.
Turn off the machine and rest the X axis on the blocks at each side by adjusting the Z axis screw. Then turn the machine back on and raise the Z axis and the X axis gantry should stay at the same height on each side.

It works with most Copymaster or CR10S style printers with dual Z axis.