An STP Pay Event is reported via STP Reporting and is one of the steps in your payroll process. Pay Events will include values for Gross, Tax and Net in the Header of the Event. You can use these values to reconcile to the Payroll Report for the period. All successful STP Pay Events will show in the ATO's business portal.
An STP Update event updates all employees' MyGov accounts and can be used to report failed Pay Events or events as part of your End of Year process. As they aren't the result of payroll processing, you won't see values against the Header fields and they won't show in the ATO's business portal.
Regardless of the type of event you report, the Employee's MyGov account will be updated with the employee's current YTD data. The Employee’s MyGov data is overridden on each event based on the PC date and time stamp.