I think Xlines is more about the overall experience rather than any one thing in particular. Yes, there are probably more routes that you could shove under the branding because they go direct (and maybe some that shouldn't be there), but do they fit the 'ethos' of the Xlines brand?
Obviously being 'express' (by whatever definition you care to use) makes up a large chunk of that experience, but there's also the improved customer experience on board in comparison to the more 'normal' routes, with the high back chairs, WiFI, USB, NSA etc. Although most vehicles 2015 onwards meet the Xlines spec regardless of the branding, so that is probably less of a factor.
RE: Sunderland District | A new network of brands for services in Sunderland