This defines where the rule should be executed. Either client or server. By default the rule will be executed in any case (client&server), never mind if JAXFront is running as a server or not.
One can use JAXFront on server side to validate incoming XML streams. Therefore only server relevant rules can be processed by setting the JAXFront engine as 'server instance' (call the method: JAXFrontProperties.getInstance().setIsRunningAsServer(true)) .