Regions allow you to split up the screen or application window into areas which can be used in actions as part of the matching criteria.
If you have a region type selected, you will see a Region group of checkboxes available for actions associated with the application. Simply check all of the regions in which the action should be matched. An example of this would be to have two actions using the same gesture. One which executes when you draw on the left half of the screen, another which executes when you draw on the right half.