Residual disability shall be defined in relation to the insured's reduction in earnings and may be related either to the inability to perform one or more, but not all, of the "major," "important," or "essential" duties of employment or occupation, or to the inability to perform all usual business duties for as long as is usually required. A policy which provides for residual disability benefits may require a qualification period, during which the insured must be continuously totally disabled before residual disability benefits are payable. The qualification period for residual disability benefits may be longer than the elimination period for total disability.