If Region is selected, all the parts of the drawing that are located within an individually defined frame stay viewable while all objects outside this region are clipped.
This is how to create a Region:
1.Select viewport for the Clipping by a simple click in the desired viewport.
2.Start tab, Clipping menu: Click on Region. The clipping Region becomes visible as a green frame in all available viewports.
3.Adjust Region: Use Object Interactor or the other interactors to move or rotate, enlarge or decrease the frame or change its proportions. The part of the drawing that will become invisible will be shown in gray in the active viewport.
4.Confirm by Enter or with a single click in the viewport. The Clipping Region is now active.