It notified the number of digits of HSN Code to be declared in a tax invoice issued by a registered person. 

78/2020-Central Tax, dated 15.10.2020 (with effect from 01.04.2021); 
90/2020-Central Tax, dated 01.12.2020.

It clarifies that the provisions of clause © of Section 31(3) of the CGST Act, 2017 shall apply to a person paying tax under Notification No. 2/2019-Central Tax (Rate), dated 07.03.2019. 

It specify certain registered persons (having aggregate turnover in any preceding financial year from 2017-18 onwards more than Rs. 500 crores) who shall be required to issue e-invoices with effect from 01.10.2020.
The turnover limit for mandatory issuance of e-invoice reduced to 100 crores w.e.f. 01.01.2021(vide notification No. 88/2020-Central Tax, dated 10.11.2020).
The turnover limit for mandatory issuance of e-invoice reduced to Rs. 50 crores reduced with effect from 01.04.2021 (vide notification No. 05/2021-Central Tax, dated 08.03.2021).
It excludes certain registered persons including a government department, and a local authority from the requirement of issuance of e-invoices. 

13/2020-Central Tax, dated 21.03.2020.
61/2020-Central Tax, dated 30.07.2020;
70/2020-Central Tax, dated 30.09.2020;
88/2020-Central Tax, dated 10.11.2020;
05/2021-Central Tax, dated 08.03.2021 
23/2021-Central Tax, dated 01.06.2021
01/2022-Central Tax, dated 24.02.2022;
17/2022-Central Tax, dated 01.08.2022.
10/2023-Central Tax, dated 10.05.2023. 

It mandated issuance of invoices with QR code by certain class of registered person (having turnover of more than Rs. 500 crores) with effect from 1.4.2020.

It mandated issuance of invoices with QR code by certain class of registered person (having turnover of more than Rs. 500 crores) with revised effective date of 01.10.2020.