ISD stands for Input Service Distributor. An Input Service Distributor refers to a person who distributes credit, in respect of the tax invoices of the services received at the Head Office, to its branches where the services have been supplied actually. Tax invoice refers to the invoice issued under Section 23 of the Model Goods and Services Act. If you are an existing ISD Taxpayer, you need to apply afresh in the GST Common Portal for the State in which you desire to seek registration. For that you need to inform your Central Jurisdictional Authority.