The PopulateDeliveryNotes order rule action means that if the Delivery Notes field is empty on the order or you don't want to use a customer's delivery notes on orders you can populate one for them. For the Param field, you enter the delivery notes you want to show on the order.
The PopulatePackingNotes (and the AddPackagingInstruction) order rule action does the same as above but instead of populating the Delivery Notes field, it will populate the Packing Notes field on the order.
Both the PopulatePackingNotes and AddPackagingInstruction order rule action populate the same field on the order.