There are several versions of the CML schema. The current version is Schema 3.
Schema 3 is a refinement of Schema 2.4. Much of the content model has been removed as have those elements and attributes which were deprecated in the previous version.
The only content model retained in Schema 3 is for elements which must be empty or elements which must contain string content. All other elements are permitted to contain any other CML element and/or and elements from a different namespace.
The last stable release was Schema 2.4 and has remained unchanged since 2005.
Schema 2.4 may be viewed here and downloaded by clicking this link. There are many examples of how to represent chemistry in CML conforming to this schema available here. All these examples should also validate against Schema 3.