(unknown charset) Applicability of the RDF formal grammer

Dear W3C Members:

I've just read through the documents on RDF model and schema.  There is one question confusing me right now.  In the 'RDF Model and Syntax Specification', section 6 gives the formal grammar for RDF.  From the BNF rules, a RDF file is defined as a list of 'obj', where each 'obj' is either a description or a container.  However, some real RDF files, such as the one shown in Appendix A of the 'RDF Schema Specification', do not follow this basic construct.  Does this mean that the BNF syntax in section 6 will not apply to certain RDF files?

Another question:
What's the difference between:
...
<rdfs:Class rdf:ID="MotorVehicle">
    ...
</rdfs:Class>
...

and 

<rdf:Description ID="MotorVehicle">
  <rdf:type resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
  ...
</rdf:Description>

Thanks a lot

Victor Liu
UCLA Computer Science Dept.