Input Service Distributor rules govern pro rata allocation of input tax credit to recipients sharing the same PAN under GST. An Input Service Distributor (ISD) is an office that distributes input tax credit to registered recipients with the same PAN, must obtain separate registration and file monthly GSTR-6. Credit may be distributed only to recipients who use the services for business and only up to available credit. Where attribution is unclear, distribution among operational recipients is pro rata based on state turnover; eligible and ineligible credits must be segregated and distributed in the same month, and adjustments on credit notes are apportioned proportionately.