XMI21 — How are attributes and compositions inherited?

In my understanding of the XMI 2.0 spec, it is not clear about how attributes and compositions are inherited. On page 34 point 1.8.7 is stated, that for attributes and compositions copy-down inheritance is required. Contrarily is rule 4c on page 57, which states that only local attributes, references, and compositions are included if the org.omg.xmi.useSchemaExtensions is set to true.