Many trace events relating to an extension that has appearance buttons, will indicate a button number against the event. When troubleshooting, this allows you to understand why, for example; a call alerted on a particular extension.
If you are tracing from the Extension Status screen, you also see My buttons marked against call state changes.
In some cases, a call may alert on more than one button on the same extension. For example, the extension might have a line appearance for the line originating the call and a coverage appearance for the destination of the call. In this case, the trace only shows the first alerting button.