Categories are used to help organize events so that they can be filtered in the Event Viewer. Each Source can define its own numbered Categories and the text strings to which they are mapped. The Categories must be numbered consecutively beginning with the number 1. With Event Viewer, it is possible to filter by Source and within Source, by Category. The security system uses several categories: Logon/Logoff, File System Access, Privileged Actions, Change in Security Policy. Categories for other parts of the system are developed over time as it becomes clear which events are being logged. An application can define its own categories.