These Notes are the Notes entered against a customer in AR-A that can be pulled into the order when it is saved.  
 
If the Bill-To or Ship-To customer has notes entered in AR-A Enter Customers you will be prompted whether to include the Notes on the order.  You can include them as is, edit them or not include them at all.  You can also designate the notes as Hidden which will include the notes on the sales order for internal information but not print them on customer documents such as the Invoice or Acknowledgement.  Each line of the Notes can be tagged as hidden or not when in Edit mode, there is a Hide Notes button which will toggle the Hidden flag for each line.
 
Note lines can also be designated as Hidden when they are originally entered in AR-A.