Is it correct that in Behavioral Driven Development, because test scenarios are derived from requirements they are optimized up front and therefore Combinatorial Test Design (CTD) is of no use?   I don't think so.  In fact, scenarios obtained from requirements may not be optimized or contain holes.   Typically, a scenario and/or user story will contain several points of variability.   Using CTD the Cartesian product implicitly implied by the user story is modeled and we make sure the number of scenarios to run are optimized and we avoid omissions.