Service tax exemption on contract-value basis limits taxable amount when commissioning and installation agencies opt for it. Exemption allows a commissioning and installation agency, at its option, to have service tax measured on a percentage of the gross contract amount for supply together with erection, commissioning or installation; the benefit is available only if the agency has not taken the alternate notification benefit for the same contract and has not availed Cenvat credit on inputs or capital goods, and the gross amount charged includes the value of plant, machinery, equipment, parts or other material sold during provision of the service.