IDREFs and IDs

In schemas, how do you ensure that a IDREF (identifier reference)
corresponds to a particular ID? According to the book I am reading, a
identifier reference will "ensure that the value of an XML attribute matches
a previously defined unique identifier" ("XML, your visual blueprint for
building expert web pages", by meranGraphics and IDG Books, page 106)

An example of an ID declaration that they provide is:
-	example:
-	<AttributeType name="product_code" dt:type="id" />
-	<ElementType name="product" content="eltOnly" model="closed">
-	<attribute type="product_code"/>
-	</ElementType>

While the IDREF is: 
-	<AttributeType name="bestseller" dt:type="idref" />
-	<ElementType name="name" content="eltOnly" model="closed">
-	<attribute type="bestseller"/>
-	</ElementType>

There does not appear to be anything saying that "bestseller" must match the
entries in "product".