I just found this sentence in the French RGS :

"§V.2.1.p : Il est recommandé que le prestataire utilise le format des tickets d’incident de sécurité proposé dans [ETSI_ISG_ISI] : Standards ETSI ISI Indicators (ISI 001-1 and Guides 001-2), ISI Event Model (ISI-002), ISI Maturity (ISI-003), ISI Event Detection (ISI-004) – 5 standards sur la détection des incidents de sécurité."

From what I understood, ISI defines a taxonomy for incident which can help classify incidents but certainly not a "ticket" definition which would be more the role of IODEF.

There is certainly a possible collaboration between those two formats but ISI can't replace IODEF.

What do you think ?