From a FDLP (Federal Depository Library Program) Announcement:
In continuing the strong partnership between the U.S. Government Printing Office (GPO) and Federal depository libraries nationwide, GPO is encouraging all interested depositories to crawl its Federal Digital System (FDsys) in order to provide distributed access to content.
In order to improve the ability for crawlers to identify and index new content efficiently and completely, GPO has implemented Sitemaps for FDsys. This is the most effective way to crawl FDsys.
The Sitemap for FDsys provides a hierarchical list of the full content available from the site and is updated as new content is made available to the public. These updates help crawlers determine what new content needs to be crawled and when it was updated, and allows FDsys to provide more information about how and when content is added and updated.
GPO will continue to maintain responsibility for managing Federal content within scope of the Federal Depository Library Program (FDLP) and for providing free and permanent public access to this information.