There are three ways of being admitted to an IISER.
SCB criteria are applicable for only the top 1% of students of each of the boards across India. To simply word it, those who have secured 99 percentile are a part of SCB. If you secure marks that are above the cut-off, you will get INSPIRE scholarship as well.
The top 1% of students from across India and all boards are invited to give the IISER test. After this, a rank list/merit list is prepared about the marks scored in the exam. And the top students are admitted to the six branches of IISERs.