Circular Viewport
Let's say you want to create a circular viewport completely contained within a larger rectangular one. You don’t want objects displayed in the rectangular viewport, only showing objects in a circular viewport.
Here is what you can do:
In a layout viewport, create a rectangle and a circle within it.
Create a copy of the circle in the same location as the original circle (use COPY 0,0).
Change all three objects into regions with the REGION command.
Subtract one of the circular regions from the rectangular one with the SUBTRACT command.
Enter the MVIEW command and specify the Object option. Select the rectangular region, change it into a viewport with a "hole" in it.
Repeat the MVIEW command selecting the remaining circular region. This converts it into a separate circular viewport, that fills the hole in the previous one