Each APS Catalog index (except for Package Resources) has a feed link. To extract the feed link from an index, parse such index for //a[@id='feedLink'].
Package Updates feed differs from other feeds by its semantics. This feed contains a list of packages that update an application, rather than a list of application packages. To indicate this fact, a client must use //a[@id=' to retrieve the Package Updates feed link from a Package index. updates']