Installation and configuring plug-in Flexible Discount Rules

The plugin creates the discount with the type "Assigned to order subtotal" - "Flexible Rule Discount". The plugin creates it itself.

The customer sees it on the shopping cart page before the checkout. This discount is for the subtotal amount.

You could configure several discount rules. The discount has a parameter "Cumulative". 
If you configured several discount rules with "Cumulative=false" then the plugin takes the largest discount for the customer.
If you configured a discount rule with "Cumulative=true" then the plugin adds the amount to the final result.