Conservation of energy and momentum are separate phenomenon and in this context, "closed" relates to the latter. You can have systems like particle collisions i

Conservation of energy and momentum are separate phenomenon and in this context, "closed" relates to the latter. You can have systems like particle collisions in a collider that receive energy from (are accelerated by) the electromagnets but the collision is a closed system because when the products scatter, they conserve the momentum of the colliding particles.