An “ancillary proceeding” (or “ancillary suit”) is one that “grows out of and is auxiliary to another suit and is filed to aid the principle suit, to enforce a prior judgment, or to impeach a prior decree.” Black’s Law Dictionary 1475 (8th Ed. 2004). In federal civil litigation, ancillary proceedings most often arise out of disputes relating to subpoenas served on third parties located outside the district where the underlying action is pending.
Under Fed. R. Civ. P. 45(a)(2), any subpoena served on a non-party must be issued from the federal district court where the non-party is located, and that same federal district court will hear any disputes relating to the subpoena. For example, if a party to an action pending in federal district A issues and serves a subpoena on a non-party located in federal district B, Fed. R. Civ. P. 45(c)(3)(A) requires that any dispute relating to the enforcement of that subpoena must be resolved by the federal court in district B.