A graded user ability to deliberately switch among distinct writing styles with AI assistance and to access registers the user could not readily produce alone.
Rated as the distinctiveness of styles a user can elicit on demand: outputs produced for several requested registers are judged for how reliably they differ from one another and match the target register.
Proposed measurement protocol (not yet empirically validated): Style-discriminability: independent raters, or a held-out classifier, attempt to assign AI-assisted outputs to their requested register; the score would be classification accuracy or pairwise distinctiveness across registers.