App Type: this is a specific case, where you need users to define the type of data that will be used by your element. For instance, for a map element, you'll probably want a field which will be 'Type of markers', and users will pick one of their types. Once you have defined such a field, you'll be able to define some fields that prompt the user to pick a field within that type. For instance, for a map, you'll need user to define which field of the type actually contains the address. As you'll need to know the data type of the field, you can restrict the field types so that it only shows addresses, for instance.