GST Registration is mandatory in the following cases:
Inter-State Sales - Sale of goods or services outside state of registration
E-Commerce Seller - Sale of goods or services online on an e-commerce platform
E-Commerce Operator - Person who owns and manages a digital platform for an e-commerce business i.e. buy and sell of goods or services over a digital network
RCM - Buyer who is required to pay tax as per Reverse Charge Mechanism- Other persons such as NRTP (Non-Resident Taxable Person), CTP (Casual Taxable Person), ISD (Input Service Distributor), Agent
Read more: When is GST Registration Compulsory?