If you try to select an object in 3DS MAX and move it with the SELECT AND MOVE tool and the UCS is greyed out….

so it just lets you move in one or two directions….is because is locked. Just press “X” on your keyboard to unlock.

Locking the UCS is quite useful when trying to move or align objects. Lets say that you are trying move different objects up and down, like on the next image the RED and GREEN BOX, you don’t want to affect its current location but just to move it up or down a bit.

Select the GREEN BOX with SELECT AND MOVE tool, click just once on the Z axis so that’s the active axis, then press “X”, that will lock de movement to just that axis so if you try to drag it will just move up or down along the Z axis.