Absolute Movement
The navigation of the plan area is fixed with the top wall defined as 0°, the left wall 90°, bottom wall 180° and the right wall 270°. So if you move your selected item left, it will move to the wall on the 90° axis regardless of your visualisation in plan, elevation or perspective.
Below in Absolute Movement when in Elevation, moving the unit on the by 150 lifts and the unit from 150 off the floor to 300.
Relative Movement
The navigation of the plan is that 0° is the direction you are facing and 90° is to your right, with 180° behind you and 270° to your left.
Below in Relative Movement when in Elevation moving the unit on the by 150 moves the unit back towards the top wall by 150.
To switch between Absolute and Relative movement modes, this is selected in the Items tab of the ribbon which is only active when you have a unit selected in the design.