Write expression for browser. Variable DISPLAY should be set for Linux slaves, it can be created by using Xvnc plugin (Check "Run Xvnc during build" option in Build Environment section). This axis is for setting the browser more generally. There can be set more versions of browser (firefox-8||firefox-5) or types of browsers(firefox||chrome). This axis enable you set only expression for browsers, but it is not for all labels (for example you can write firefox||explorer, but you can not write firefox&&linux). For non-browser labels, please use label expression or slave option.