You can query core, plugin, and themes individually, or check all at once.
It defines four new methods:
- xmlrpcCoreUpdateCheck – checks core, returns true/false
- xmlrpcPluginUpdateCheck – checks plugins, returns true/false
- xmlrpcThemeUpdateCheck – checks themes, returns true/false
- xmlrpcUpdatesCheck – checks all of the above, returns an array of trues/falses.
The plugin was created primarily for WPUpCheck (http://trog.qgl.org/wpupcheck/), a simple Windows application that remotely polls a WordPress site to see if updates are available.