SalesOrder.MasterUpdate performs a number of “pre-checks” before it ultimately calls Update. The MasterUpdate method was added to improve the overall UI performance experience so instead of having the UI make numerous calls to the Server and then call Update, the MasterUpdate method does the work on the Server.
Your BPM on Update should still be fired but depending on what you are doing, you may need to adjust it.
Epicor ERP Trivia - the Sales Order BO was the first one to receive this special save method and is the only BO with a MasterUpdate method name. All the other BOs that also had this technique applied have a method called UpdateMaster. Why? Because alphabetically Update and UpdateMaster sort out together and it is obvious to see that there is an UpdateMaster method on the BO without having to scroll the list of Methods.