BPMN21 — Unclear how to add new Artifact Types

The specification says about Extension:

It provides a set of extension elements, which allows BPMN adopters to attach additional attributes and elements to standard and existing BPMN elements.

In other words, extension can only add something to existing elements, but not create new element types.
However about Artifacts it says:

A modeler or modeling tool MAY extend a BPMN diagram and add new types of Artifacts to a Diagram.

Given the extension mechanism, it is unclear, how new types of Artifacts can be added. And since the Metaclass Artifact is abstract, it is not possible to use the generic Artifact and extend it to create something resembling a new type. A modeler would always need to specify the concrete subclass, such as Group or TextAnnotation and thus inherit attributes and semantics he probably doesn't need.
So either the sentence is wrong, or there is another extension mechanism, that needs some explanation here.