Explanation
Action messages are generated by the master planning calculation in response to changed requirements. 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.
Actions that are related to components of bills of material (BOMs) must be applied before the actions of their parent items, because further orders that are related to higher-level BOMs might be affected.
Reference: https://learn.microsoft.com/en-us/dynamics365/supply-chain/master-planning/action-messages