The system watches for a visitor to perform an action on the site (for example, visit a given page) that causes an active slot to request content items. See Using Slots for more information.

You must specify the slot that you want to use to trigger this element. To do this, use the By Slot parameter within this element to select an existing slot. Example: Items requested by slot TodayOnly. This example tells the system to watch for a visitor to perform an action that causes a slot named TodayOnly to request content items.

Bear in mind that in a multisite environment, the slot may exist on more than on Web site.