The term 'Levels; Classes' can be summarized by the crossword solution 'STRATA' (6 letters). STRATA is a term used in geology and other sciences to refer to layers of sedimentary rock or soil that have been laid down over time. It is also used to describe social, economic, or political divisions in society. In this context, 'Levels; Classes' could refer to the different levels or classes of people in a given society.