Add Opacity to Polygon settings
This is somewhat doable with Patterns, there being a series of patterns that range from essentially low coverage/clear to high coverage/solid. What would be ideal, however, is an Opacity option of the desired pattern. For instance:
Fill Properties (of a Polygon):
Background color: Transparent
Application of this would be for a visual presentation of Probability of Area, how likely it is for the search subject to be in that area. Would be able to set some Polygons (acting as search segment boundaries) as 90%, some at 75%, some down at 50%, and barely visible at 10% for low POA, etc.