Success and failure summaries are specified using the atg.successMessage and atg.failureMessage parameters. For example, to specify message summaries with a form you could add the following:

<input name="atg.successMessage" type="hidden" value="Updated info
  for John Doe."/>
<input name="atg.failureMessage" type="hidden" value="There was a
  problem updating John Doe's info."/>

If no failure summary is specified in the request, the messaging UI adds the default summary that is specified in the message bar:

<div dojoType="messaging:MessageBar" errorMessage="There were
  errors with the request."/>

By default, no message is displayed for successful requests.