WS-Policy Attachment specs, chapter 3.4 on external policy attachment:

"This mechanism allows Policies to be associated with a Policy Subject independent of that subject's definition and/or representation through the use of a <wsp:PolicyAttachment> element. This element has three components: the Policy Scope of the attachment, the Policy Expressions being bound, and optional security information. The Policy Scope of the attachment is defined using one or more extensible domain expressions that identify Policy Subjects, typically using URIs. [...]"

This is supposed to use WS-Addressing to deal with policy scopes.