Multiple delivery addresses allow you to specify a different delivery address for each order line. When posted, a journal listing the relevant order lines is produced for each delivery address specified.
If you clear one of the check boxes, when posting the item, the delivery address is that on the form header. This is either the customer address or an alternative address you can specify with the button, option in the form header.
If you select one of the check boxes, the delivery addresses are those specified for each sales order line. By default, this is the customer address (unless you have chosen an alternative address by clicking the button, and selecting the option in the form line). Items are produced for each delivery address, listing the relevant sales order lines on each item.