Customizing Activities sub-panel

I plan to add a shortened description field to the Activities sub-panel. I am copying the ForActivities.php files from the modules/ <Calls/Meetings/Tasks/Emails>/metadata/subpanels directory into the matching
custom/modules <Calls/Meetings/Tasks/Emails>/metadata/subpanels directory and then make my additions to the file.
So I am rolling along with this change and I see that there is no ForActivities.php file for Emails. I do see a ForHistory.php file.
Does anyone know how the Email information gets displayed on the Activities subpanel and what file I need to change to add my new field?