While the OpenAPI Specification tries to accommodate most use cases, additional data can be added to extend the specification at certain points.
The field name MUST begin with x-, for example, x-internal-id.
The value can be null, a primitive, an array or an object.
When the value is an object or an array of object, the object's fields names do not need to start with x-.
The extensions may or may not be supported by the available tooling, but those may be extended as well to add requested support (if tools are internal or open-sourced).