At line 857 of the configuration.adv tab in Marlin 2.0.9.1 we find Z_stepper_align_XY. And this is where we set the probe coordinates which are expressed as { X, Y}. Because this feature will support 2, 3, or 4 steppers controlling Z Marlin come pre configured for middle of the road with 3 sets of coordinates. If we are using 2 steppers we can only adjust for one axis and so we only need 2 probe points and can just delete the third set as I have done below..



I am setting up a 4 Z stepper motor CoreXY and that will use 4 probe points so that would look like {{50, 50}, {460, 50}, {460, 460}, {50, 460}}. Because my print bed is 510x510 and I want to leave a bit of a buffer for the Z probe offset. Each probe point is a place to make a correction with a stepper motor. So the amount of probe positions should always mirror the amount of independently controlled stepper motors in use.