A quick Google search didn’t yield a quick explanation of this, so I’ll quickly describe the difference.
Alert Generating Rule:
A rule setup that generates an alert that shows up in the active alerts. It stays in a “New” state until it’s modified by a script or manually closed.
A rule setup that collects information and can be displayed via the “Event” view. These rules don’t generate alerts but allow for reporting via the event view and via the datawarehouse.