jsonapi-rb works by defining serializable resources for your business objects, which describe how to serialize a model of a given type in a given serialization context.
The renderer then builds a JSON API document from your business objects. In order to do so, it needs to find for each type of model the corresponding serializable resource class, instantiate it, and then combine all the JSON API representations of isolated resources.
Some context can be forwarded to the serializable resources: the exposures, specified when calling the renderer, are available within the serializable resource as instance variables.