It’s a bit messy but the rule in question might be from
the user’s instance
the community’s instance
the community itself
Typically when mods ban you based on violations of some global rule they’ll highlight it, e.g. “global rule 1”. And it’s good form to include the short description of the rule being violated as the reason for the ban (e.g. instead of “rule 1”, saying “rule 1 - uncivil behaviour”).
Without further clarification, however, it’s usually the community rules. Otherwise you’ll likely get banned from your account (if you violated the global rules of your instance) or from the instance where the comm is.