Custom Apps can be initiated by emails. Emails can either be free form (sent by people) or structured (generated by filling a web form).
Direct Interface
This helps configure your app items so that they can be initiated by a simple email. Each custom app has a unique email address to which the email must be sent. A mail sent to this email address will trigger off the app item. For example you may have a Help-desk app that you wish to be triggered by a person sending mail to a specific email address.
Indirect Interface
This helps you integrate an existing system with one of your custom apps. For example you have a web based form on your web site that users fill and submit to initiate an App in Celoxis.