This is the second version of this working draft and from the "Purpose of the Ontology and the API" section I've extracted the following:
The ontology will define mappings from properties in formats to a common set of properties. The API then will define methods to access heterogeneous metadata, using such mappings. An example: the property
createDate from XMP can be mapped to the property
DateCreated from IPTC. The API will then define a method
getCreateDate that will return values either from XMP or IPTC metadata.
An important aspect of the above figure is that everything visualized above the API is left to applications. For example.
languages for simple or complex queries
analysis of user preferences (like "preferring movies with actor X and suitable for children")
other mechanisms for accessing metadata
The ontology and the API provide merely a basic, simple means of interoperability for such applications.