Structurizr's C4 model implementation has well-defined semantics and rules (e.g. you can't add components to a System Context view), which works for most use cases. But there are times when you might want to do something a little different, and not be restricted by these rules. For example, perhaps you want to show how users have access to deployment nodes, or that your software system is used by piece of hardware. For this reason, you can define your own custom model elements, and apply whatever semantics you like through the existing visual styling mechanism of tags and styles.
Custom elements reside at the top level of the model only (there's no support for creating hierarchies of custom elements), and they can be included on any diagram. Custom elements can have a name, type, and description. Since custom elements use the existing tagging and styling mechanism, any custom elements used on a diagram will be automatically included on your diagram key/legend. Custom elements can also be used on custom diagrams.
Please note that, at the moment, custom elements are only supported via the Structurizr for Java client library, and the Structurizr DSL.