SPARQL-SD question concerning

Hi,

I'm having some difficulty understanding the intention of the SPARQL
Service Description property sd:availableGraphs.

The current Working Draft says sd:availableGraphs "relates an instance of
sd:Service to a description of the graphs that may be used in the
construction of a dataset either via the SPARQL Protocol, with FROM/FROM
NAMED clauses in a query, or with USING/USING NAMED in an update request."

Aren't the graphs, which one can refer to with FROM/FROM NAMED clauses in a
query, the sd:names of the sd:namedGraphs of a sd:Dataset? Is
sd:availableGraphs only an alternative way to describe the same fact?

Kind regards,
Marcel Otto