Index Generator is able to exist autonomously from the data it collects and queries it receive. While Index Generator is usually thought of as holding a single collection, it may itself federate multiple collections. Index Generator may be specialized for particular data types, ranging from plain text, to formatted text (SGML, XML, HTML…) to multimedia. The only fixed requirement for an Index Generator is that it be able to respond to a query with a response set, within the security
guidelines described below.