Browser versions targeting allows you to set your line items to target not only certain browsers, but also their specific versions.
You can include browsers and their versions into targeting and exclude them. When you target a browser, you can exclude some of its versions from targeting at the same time. Likewise, you can exclude a browser from targeting while targeting certain versions of this browser.
By default your line items target all browsers and browser versions.
Browser Version reporting keys to analyze performance in u-Slicer.