To start with, let's make a simple scene. The image above is what I like to call a "quick and dirty"--something patched together in five minutes for demonstrative purposes. It's a door. No, really, it is. And that little...circle thingy...is a rug. So. Moving on--for this scene we're going to make the doorknob clickable; let's say hypothetically that clicking the doorknob would "open" the door and play a new scene.
But we don't want to make the lines and fills for the doorknob oval itself clickable; it makes editing the doorknob itself more difficult later, and also limits the clickable area. We want to expand the area to the doorknob and a small area around the doorknob so the user doesn't have to try so hard to click right on that one tiny, exact spot.