ISB does not accept CAT score for admission to its flagship PGP and other MBA equivalent programmes.
The candidates are required to appear for GRE/GMAT and apply with those scores. The benefit of getting these scores is that they are valid for multiple years, while CAT score is valid for only one year. So while the admission to IIMs through CAT scores needs to be done the very year you are offered a seat, you can appear for GMAT earlier and then apply for MBA in subsequent years. Hence you get the option to defer your decision of MBA for a while.
ISB does accept CAT score for its Fellow Program in Management (FPM)