Extension: a Reference Schema that also carries refinements.

{ "type": "hm://…/example/person", "properties": {…} }

An extension is a subtype. It has the parent's fields plus the new ones, and each new field is a property that says whether it is required.

There is no extends keyword. type names the parent. Any other key on the node is what makes it an extension instead of a bare Reference Schema.

Example: employee extends person.

See also

References and naming: include, link and extension side by side.

Include schema: the variant an extension belongs to.

Property: the fields an extension adds.

Generic: the other way to reuse a schema.