Content models define the structure and content of data objects in a formal way so that the content of information exchanges can be understood, used and validated in a consistent and repeatable way.
The structure and content of data objects is defined in schema which are defined when an information exchange is designed (or redescigned).
The messages in an information exchange contain objects that are instances generated from the schema by the sender of the message. At any time, if needed, instances may be validated against the schema by either the sender or receiver of the message.
In NIEM information exchanges, instances are referred to as Information Exchange Packages (IEPs) and schemas are essential artifacts of an Information Exchange Package Documentation (IEPD) that describes an information exchange. Other artifacts in an IEPD typically include sample instances, use cases, business rules, and other business and technical documentation guiding the implementation of the information exchange. Each IEPD should also include metadata about the exchange including the creator, the current version and the change history.
NIEM supports JSON information exchanges including IEPs in JSON format and IEPDs that include JSON Schema
More detail on NIEM models and their implementations in XML and JSON, including NIEM concepts, content, and releases, is available.