Custom apps can provide API that can be used when generating emails.
for example you might have an app that provides a function to generate a link to a users social media. The app name might be socialLinks and the method to call is called "genSocialLink" and you pass it a UserResource object. You would call the method like this:
When the email gets generated it will then run that method and use the returned value as part of the email which might look like this: