Fi and Jupiter are debit cards - most people have those. Yes I know, Fi and Jupiter have also launched Federal Bank CCs but most don't have those since those are paid. Federal Bank allows their core CC if you have a Fi or Jupiter DC.
OneCard by Fed bank users are unfortunately out of luck since it is a true credit card. Similarly if you've got Scapia, Fi CC or Jupiter Edge CC you can't get Federal core CC. If you've got Jupiter RuPay CC by CSB, you can get a Federal core CC.
Another strict rule of Federal Bank's approval/rejection criteria is you must have very less credit enquiries in the last 6 months before you apply. They decline if you look as credit hungry to them with too many enquiries in too short a period.