Writing

I think they are part of the spec, e.g.:

> 3.8.1.1 mix:referenceable
> 
> [mix:referenceable]
> 
> mixin
> 
> - jcr:uuid (STRING) mandatory autocreated protected
> 
> INITIALIZE
> 
> This node type adds an auto-created, mandatory, protected STRING property to the node, called jcr:uuid, which exposes the identifier of the node. Note that the term “UUID” is used for backward compatibility with JCR 1.0 and does not necessarily imply the use of the UUID syntax, or global uniqueness.