Motion > Directional Modes > View Dependent, Level
Defines the forward, back, left, and right motions based on the direction in which you are looking. The eye point to center point vector of the view cone defines this direction. Using the Forward/Backward, Left/Right mouse drag mode, forward moves you towards where you are looking. Back moves you away. However, your elevation in the model does not change if the view cone is pointed up or down.
The View Dependent Level mode allows you to move toward where you are looking but keep the same elevation. The mouse drag modes, positioning modes, and Position Control toolbar all affect how the directional modes operate.
Click Motion > Directional Modes > View Dependent, level.
You can also click View Dependent Level on the View toolbar.