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:

@{applications.socialLinks.call("genSocialLink", user)}

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:

<a href="http://www.facebook.com/TheUserName">Follow Me</a>