Inter partes efficacy refers to the applicability and legal effect of an act, contract or provision that affects only the parties directly involved in its creation or agreement. In other words, this type of effectiveness is limited to the persons or entities that actively participated in the formation of the act or contract in question.
When something has inter partes efficacy, it means that only the parties who agreed to or participated in that action or agreement are legally bound by its terms and conditions.
Inter partes efficacy is common in contracts, business agreements, and other legal commitments in which the parties agree to specific terms and conditions that apply only to each other. Therefore, this efficacy is restricted to the parties that have a direct link to the act or contract in question, without affecting third parties that are not involved in the creation or agreement.