As noted in Bug #886820, while Beaker's system search capabilities are powerful and flexible, it isn't always clear how to go from a question like 'I need a system with a graphics card' to '<device type="VIDEO"/>', or, more surprisingly, from 'I need a system without a graphics card' to '<device op="!=" type="VIDEO"/><device type="NETWORK"/>' (The requirement for a network interface to be present rules out systems which haven't been inventoried at all). The docs should include some example of how to handle certain kinds of query, such as "I want a specific model of network card" or "I need a graphics card" or "I need there to be no graphics card".
On Gerrit: http://gerrit.beaker-project.org/#/c/1795/
This change is now included in the development version of Beaker's documentation: http://beaker-project.org/docs-develop/user-guide/job-xml.html
Beaker 0.12 has been released.