Which are the cases in which registration is compulsory?

As per Section 24 of the CGST/SGST Act, the following categories of persons shall be required to be registered compulsorily irrespective of the threshold limit:
i) persons making any inter-State taxable supply;
ii) casual taxable persons;
iii) persons who are required to pay tax under reverse charge;
iv) electronic commerce operators required to pay tax under sub-section (5) of section 9;
v) non-resident taxable persons;

vi) persons who are required to deduct tax under section 51;
vii) persons who supply goods and/or services on behalf of other registered taxable persons whether as an agent or otherwise;
viii) Input service distributor (whether or not separately registered under the Act)
ix) persons who are required to collect tax under section 52;
x) every electronic commerce operator
xi) every person supplying online information and data base retrieval services from a place outside India to a person in India, other than a registered person; and,
xii) such other person or class of persons as may be notified by the Central Government or a State Government on the recommendations of the Council.