Kafka Topics are basically message bodies and as such can contain virtually any type of data. In the case of Topics populated by the Connect CDC (SQData) Apply Engine they generally represent either the complete before and after image of the source data from the Capture Agent or only selected fields from the Captured data that might represent a particular business event. The Payload Format type for Kafka topics may be either AVRO or JSON.
The Kafka Apply Engine script generated in this step is a no-map replication script where the source datastore and target topics contain the identical data. If you have a use case that involves replication of non-identical source/target datastores/topics, you will need to create Kafka target descriptions before creating the Apply Engine.