This is an overview over most models exposed by the different API. It does not explain every single property of all models, but will try to give a solid introduction to the important parts.
The application metadata document holds technical information about the app and the data type requirements.
An instance is a metadata container that is used to track the status and progress of one interaction between an app user and owner.
A data element is a metadata container that is used to track the status of actual data blobs.