The answer is given in WWDC's keynote on iPhone web dev.
At 33 min and 06 seconds in the WWDC 2007 "Designing Web Content for iPhone" (this is available for free for anyone who has an Apple Dev Connection account).
Action means that the region is either :
- an Image map area (remember those old school hot zones in a big image...)
- an elements that will trigger "mousemove, mousedown, mouseup or click" javascript events
There you go ! That explains why you don't see it everywhere.