Look at the Notifications server side script in the template.
It has method notifyAboutItemChanges_ which is passing the data to this record.
function notifyAboutItemChanges_(changes) {
var settings = getAppSettingsRecord_()[0];
if (!settings.EnableEmailNotifications) {
return;
}
var data = {
appUrl: settings.AppUrl,
itemType: changes[0].Type,
itemKey: changes[0]._key,
itemName: changes[0].Name,
modifiedBy: changes[0].ModifiedBy,
changes: changes
};
// Email subject.
var subjectTemplate =
HtmlService.createTemplate(settings.NotificationEmailSubject);
}
This function is passing this data to your settings record.
So no magic here :) You need to pass the data to your record which will be replaced at run time with the values.
For more details on Email refer this sample app.