What is the Assertion Query and Request Protocol used for?

It defines messages and processing rules to fetch assertions that already exist by their reference (ID), or to search for assertions using query parameters such as a subject and the attributes wanted.

Not every SAML exchange is a browser login. Sometimes an SP already knows who the user is and needs more information later, or holds only a reference to an assertion. The protocol offers:

• <AssertionIDRequest>: "give me the assertion with this ID" (retrieval by reference).
• <AttributeQuery>: "for subject X, tell me attributes A, B, C" (for example the SP asks the IdP for a user's current role).
• <AuthnQuery>: "which authentication statements exist for subject X?"
• <AuthzDecisionQuery>: "may subject X perform action Y on resource Z?" (frozen, like the matching statement).

These requests are typically sent server to server over the SOAP binding, without involving the browser.

Go deeper:

• SAML 2.0: Assertion query/request profile (Wikipedia) — an attribute query and its response as XML.