clarification needed for include

Hi,

Section 4.2.1 in XML Schema Part 1 2nd Ed mentions
in the "schema representation constraint" section that:

"1.2 It resolves to a <schema> element information item
in a well-formed information set, which in turn corresponds
to a valid schema."
(In http://www.w3.org/TR/xmlschema-1/#layer2)

It basically asserts that the schema document being included
has to be "valid". I am not so sure what this "valid" means.

Is it "valid" in terms of what? I guess it is probably meant to be
"valid" against the whole XML Schema specification.

If it is the case there's another question.

Say for example, schema A includes schema B, and B uses
a component (eg. type definition) that is defined in schema A.

My question is: does schema B have to also "include" schema A?
If B does not include A, then schema B is now "invalid" since it does
not validate as a stand-alone schema document because some of
the references (eg. type references) do not (cannot) resolve without
being aware of components in  schema A.

I know I can experiment with the implementations, which I did including
ours and got different results. Only one implementation required
schema B to include A, but others did not care about it.

Any comments are appreciated.

Thank you,

-Takuki Kamiya (Fujitsu Software Corporation)