Each application (event source) can define its own numbered categories and the text strings to which they are mapped. The Event Viewer can use the category to filter events in the log. The categories must be numbered consecutively beginning with the number 1. The category number is specified by the EventInstance.CategoryId property. The Event Viewer can display the category as a numeric value, or it can use the category as a resource identifier to display a localized category string. For more information, see System.Diagnostics.EventInstance.