(21)National rules concerning the participation of self-employed persons in IORPs differ. In some Member States, IORPs can operate on the basis of agreements with trade groups whose members act in a self-employed capacity or directly with self-employed and employed persons. In some Member States, a self-employed person can also become a member of an IORP where the self-employed person acts as employer or provides professional services to an undertaking. In some Member States, self-employed persons cannot join an IORP unless certain requirements, including those imposed by social and labour law, are met.