the rules repeater behavior creates a repeat region that is controlled by a rule set in your eCart object such as discounts or shipping.
to see a good example of the rule repeater behavior in action, add a cart display manager to a page, on the third step of configuring the cart display, set the shipping, discounts, tax and calculations to show individually, this will use the rule repeater behaviar to show each rule that applies to the cart as an individual line.