When should bill of supply be issued instead Tax Invoice under GST ?

As per the Revised Model GST Law , Bill of supply should be issued by a supplier in the following cases:
 The supply is that of exempted goods or services; or
 The supplier has opted to pay tax under composition scheme

Relevant Extract of Revised Model GST Law

As per clause 3(b) of Section 28 – Tax invoice–  of Revised Model GST Law  A registered taxable person supplying exempted goods and/or services or paying tax under the provisions of section 9 [Composition Levy]

shall issue, instead of a tax invoice, a bill of supply containing such particulars and in such manner as may be prescribed:

PROVIDED that the registered taxable person may not issue a bill of supply if the value of the goods or services supplied is less than one hundred rupees except where the recipient of the goods or services requires such bill.

When Bill of Supply not required to be issued under GST

No. A ‘Bill of Supply’ need not be issued if the value of the goods or services is less than Rs 100 unless the recipient demands a bill.
However, in such cases, a consolidated ‘Bill of Supply’ should be prepared at the close of each day in respect of all such supplies. ( Third Proviso to Rule 3 of GST Draft Invoice Rules released by CBEC )

Bill of Supply Format

As per Rule 3 of GST Draft Invoice Rules released by CBEC , Bill of supply A bill of supply referred to in the second proviso to section 23 shall be issued by the supplier containing the following details:-

(a) name, address and GSTIN of the supplier;

(b) a consecutive serial number containing only alphabets and/or numerals, unique for a financial year; (c) date of its issue;

(d) name, address and GSTIN/ Unique ID Number, if registered, of the recipient;

(e) HSN Code of goods or Accounting Code for services;

(f) description of goods or services;

(g) value of goods or services taking into account discount or abatement, if any; and

(h) signature or digital signature of the supplier or his authorized representative:

Provided that the proviso to sub-rule  (1) of rule 1 shall apply, mutatis mutandis, to the bill of supply issued under this rule:

Provided further that the registered taxable person may not issue a bill of supply if the value of the goods or services supplied is less than one hundred rupees except where the recipient of the goods or services requires such bill:

Provided also that a consolidated bill of supply shall be prepared by the registered taxable person at the close of each day in respect of all such supplies where the bill of supply has not been issued in terms of the second proviso.