I think Classifications are provided for those variables that are populated by the customer's implementation. Values in Dimensions like Browser, Countries, etc. are automatically derived. Like, Browser report is populated using information from the user agent, Countries information is populated using the data shared by Device Atlas and so on. They aren't populated using the implementation on the page.
I didn't mention out of box variables. What I meant was the variables which are populated in the AppMeasurement implementation can be classified. While the variables which are automatically populated by Adobe can't be classified.
Variables like campaign, product, channel are populated using s.campaign, s.product, s.channel variable in the implementation. But we don't have any variable to populate Countries or Browser.