Tax Invoice: Every registered taxable person under GST supplying Goods or services is required to issue a tax invoice for all supplies effected. The word "Every registered Taxable person" clearly specifies that issuing Tax Invoice is compulsory under GST law. However, government may notify some other document for certain category of services. E.g. Bus ticket, Bank Voucher etc.,
Bill of Supply: A supplier supplying exempted goods or service or a supplier who has opted for composition levy scheme has to issue a bill of supply instead of a tax invoice. A bill of supply is not eligible for claiming input tax credits.