For an Auto Plane feature, the Display list determines how the plane will be displayed in the Graphic Display window.
Selecting None will not draw the plane feature even though it's still created in the measurement routine.
Selecting Triangle will cause the plane to appear as a triangle symbol around the region where the plane hits will be taken. The size depends on the hit locations.
Selecting Outline will cause the plane to appear as a square or rectangular outline around the region where the plane hits will be taken. The size depends on the hit locations.
See "Using the Display Area" under the "Constructing a Plane Feature" topic for similar examples of these display types.
When you create an Auto Plane feature, it remembers the last Auto Plane feature and defaults to the last used display state.