http://www.xulplanet.com/tutorials/xultu/advrules.html http://www.xulplanet.com/reference/elemref/ref_conditions.html This element should appear directly inside a rule element and is used to define conditions for the rule. Within the conditions can be placed content, member and triple elements. These may have attributes whose value is a variable name beginning with a question mark (?). When evaluating the rule for a particular RDF resource, the variables are replaced with values from the resource. If all variables can be replaced, the rule matched.