Categories can help organize your email analytics by enabling you to “tag” emails by type.
Just as you can view the statistics on all your activity under Statistics, you can go a step further and view the statistics broken down to a particular category.
The events that can be associated with category include:
The actual statistics included vary depending upon the set of enabled Apps. Emails sent, bounces and spam reports will always get tracked. Unsubscribes, Clicks, and Opens require that the associated App is enabled. Check out the Apps tab to see which apps you have enabled.
Read more about categories in the SMTP API section.