22 CFR § 92.54 - “Letters rogatory” defined.

In its broader sense in international practice, the term letters rogatory denotes a formal request from a court in which an action is pending, to a foreign court to perform some judicial act. Examples are requests for the taking of evidence, the serving of a summons, subpoena, or other legal notice, or the execution of a civil judgment. In United States usage, letters rogatory have been commonly utilized only for the purpose of obtaining evidence. Requests rest entirely upon the comity of courts toward each other, and customarily embody a promise of reciprocity. The legal sufficiency of documents executed in foreign countries for use in judicial proceedings in the United States, and the validity of the execution, are matters for determination by the competent judicial authorities of the American jurisdiction where the proceedings are held, subject to the applicable laws of that jurisdiction. See § 92.66 for procedures in the use of letters rogatory requesting the taking of depositions in foreign jurisdictions.