There are three types of messages that are generated in a GUI application: Error messages, Confirmation Messages and Notification Messages. An error message and a confirmation message indicate that an error has occurred or that a process will be or have been completed. A message that would state, "Zip code is required" would be an example of an error message. A message that would state, "Are you sure you want to delete the customer?" is an example of a confirmation message. Neither of these types of messages is treated as a unique External Output, but they are treated as data elements for the appropriate transaction.
On the other hand, a notification messages is a business type message. It is the basis of processing and a conclusion being drawn. For example, you may try to withdraw from an ATM machine more money than you have in your account and you receive the dreaded message, "You have insufficient funds to cover this transaction." This is the result of information being read from a file regarding your current balance and a conclusion being drawn. A notification message is treated as an External Output.
Notification Messages may be the result of processing and the actual processing or derived data my not be seen. If a message is created to be sent to a pager (beeper) at a given time. This is much like an alarm. That is current time is compared to set time and they are equal the message is sent. The pager message has one data element the text message.