A GSTIN (GST Identification Number) is a 15-character alphanumeric code assigned to every registered taxpayer. The first two digits are the state code, the next ten are the PAN of the business, and the remaining digits encode the entity number within the state and a check character.
Because part of the GSTIN is the state code, a business registered in multiple states has a separate GSTIN for each. Validating a customer or vendor GSTIN before billing helps avoid input-tax-credit problems later.