That which decides how a thing behaves and directs it. A controller holds authority rather than doing the work.
Medieval Latin contrarotulus (a duplicate register), from Latin contra- (against) + rotulus (a small roll, from rota, wheel); that which decides how a thing behaves and directs it, rooted in the checking of one account against another.
That which decides how a thing behaves and directs it. It holds the authority; the work of actually doing belongs elsewhere.