Source specialization for a request that originated from another Blue document or document-processing session. It is used when one document asks another document to perform an operation, provide data, bootstrap state, or respond to a pending action. Document Request is a source category, not an actor category. The actor may be a principal or agent; the source says the action was mediated through another document. This is useful for policies that allow document-to-document automation for some operations while requiring browser-session principal confirmation for others.