GST setoff current rules

DEAR CHARTERED ACCOUNTANTS,

Plse check whether I am correct or not,

1. Both CGST and SGST can't set off each other...

2.For IGST set off IGST credit first and to utilize to CGST,SGST and UTGST..

3.For CGST set off first CGST credit and to utilize to IGST UTGST

4.For SGST set off first SGST and to Utilize to IGST and UTGST


please check this is correct or not there is any mistake please clarify with explanation

Note: why the all should be set off first there usage ??