GST registration and input tax credit rules govern inter state supply obligations and reverse charge recoverability. GST registration is required for inter state suppliers unless exclusively supplying nil rated or exempt goods; tax on supplies from unregistered suppliers is payable under the Reverse Charge Mechanism and may be claimed as Input Tax Credit if eligible. The Composition Scheme has categorical ineligibilities and cannot be opted into mid year; composition dealers cannot claim ITC and must self invoice for purchases from unregistered persons. Transitional provisions permit carry forward or deemed credit for duty paid stock where documents exist and prescribe credit migration and set off hierarchies under CGST rules.