The action message that you should enable is Decrease. Here is the explanation:
* An action message is a system-generated suggestion to change an existing planned, approved, or firmed order. Action messages are generated by the master planning calculation in response to changed requirements1. For example, the ship date or quantity is changed on a sales order after you've already created a purchase order to fulfill the demand for that sales order. In this case, the master planning calculation generates one or more action messages that suggest that you update the purchase order. You decide whether to make the changes that are suggested.
* The Decrease action message indicates that production orders, purchase orders, and other receipt transactions should be decreased to prevent excess inventory levels1. This action message is generated when there is less demand for a product than was previously planned. For example, if several sales orders are canceled, the master planning calculation will generate a Decrease action message for the corresponding planned production orders.
* To enable the Decrease action message, you must select it on the Coverage groups page for the coverage group or item that you want the message to apply to1. You can also specify a decrease margin, which is the minimum percentage difference between the current quantity and the suggested quantity that will trigger a Decrease action message1.