The latest version of the Laws of Duplicate Contract Bridge defines a convention as a call that, by partnership agreement, conveys a meaning other than willingness to play in the denomination named (or in the last denomination named), or high-card strength or length (three cards or more) there.
All ACBL events are "governed" by the appropriate convention chart which lists those conventions permitted in the event. Conventions not included on the chart are not permitted in the event.
Part of the "right" to use a convention is the responsibility of deciding when it applies in probable auctions. The opponents may be entitled to redress if you did not originally have a clear understanding with your partner of when and how to use a convention you are playing.
For example, a partnership that chooses to play conventional bids over opponents notrump opening bids is expected to have discussed at least the following:
We all occasionally encounter situations where we are not sure what partner's bidding means. There exists an added responsibility if that uncertainty arises from a convention you and your partner have agreed to play. In these situations, you should tell your opponents all you know. Sometimes, the director will even ask you or your partner to step away from the table so that the opponents can talk openly with the remaining player.
Actively ethical players do everything possible in these situations to bring their opponents back to even terms -- to remove any possible disadvantage accruing to them from their side's failure to have a complete conventional understanding.