Cost accounting :is mainly concerned with data captive recording accumulation &interpretation of information cost of prodution so as enable management to focust & relevent decision. Management Accounting : is amalgamation & processing of historical & focusted data so as to make rational future plans.
since fixed cost doesn't change whether you decide to produce or not to produce...eg rent thus in marginal costing fixed cost element is removed so as to arrive at contribution... hence fixed cost is irrelevent cost in marginal costing