As per Notification No. 11/2017-Central Tax (Rate) services falling under HSN/SAC 9988 are related to job work in specific cases and they are Manufacturing services on physical inputs (goods) owned by others. It clearly means that the job worker is not supposed to add any material on his side if he adds then it will be a supply. These services attract central tax rate of 2.5% and other services at 9%. Details are given in https://indiagstdotin.wordpress.com/2017/07/27/services-by-way-of-job-work-falling-under-hsn-sac-9988/
As per Transitional Rules, if a job worker or agent is not registered under GST, then the principal has to file FORM GST TRAN-1 within 90 days from the appointed date, the closing stock held in form of inputs, semi-finished goods or finished goods.
As per the Draft Input Tax Credit Rules, details of the delivery challan sent to a job worker and received back from the job worker in a tax period has to be shown/submitted in the outward monthly return FORM GSTR-1.