Roman authors on colloquial language

This chapter discusses how Roman authors conceived of Latin as including options between which speakers and writers had to choose. This question encompasses several more specific ones. What awareness do authors show of a gap between formal and less formal varieties of their language—for example written and oral or official and familiar? What, if any, legitimacy or correctness did they assign to the different varieties, especially the informal ones? What do we learn from the metalanguage of scholars describing linguistic varieties?