I believe that ISE trust certificates whereas it is signed by CA included in "Trusted Certificates" (Administration > System > Certificates > Certificate Management > Trusted Certificate).
I configured ISE internal CA and tried to export ISE ca certificates and import them as trusted certificate. But ISE shows attached error message and I can not import that certificate. I think this is because when enable ISE internal CA, that CA certificates are stored as "trusted" by default and there is no need to import manually. Can you please let me know whether my understanding is correct?