The general requirement for most states is practice on one's area of professed competency. In states that license/register by discipline, an engineer must be licensed in that discipline to sign/seal such plans (such as for MEP). In states that do not license by discipline (I think this applies to most states), then the person must only sign/seal in their area of professed competence. For instance, if a graduate civil engineer takes the Civil exam and practices structural engineering for many years, then in areas of non-discipline registration would likely recognize that indivual as competent to practice structural engineering; however, other states (Illinois, California, etc.) might require (depending on building type/size)licensing as SE not CE.