No special rules. Just be mindful of your instance rules and community rules, but I myself have never run into surprising edge cases.
The rules for your post are the union of:
- The instance hosting your account to which you agreed when you registered.
- The instance hosting the community to which you are posting, which all communities on that instance are supposed to follow.
- The rules of that community.
Itβs like posting to a mailing list. You have to obey the rules of your email provider, the service hosting the mailing list, and the list itself. Thatβs about it. In practice, this works very well and mostly reduces to the rules of the community which are conveniently provided in the sidebar or community description.