Starfleet Regulation 191, Article 14, which gives overall command in a combat situation to the captain of the ship with tactical superiority, cited by Janeway to Captain Ransom.
Starfleet Regulation 191, Article 14 says that in a combat situation involving more than one ship, command falls to the vessel with tactical superiority. It is one of the few numbered regulations quoted in full aboard Voyager.
The rule appears in Equinox. When Voyager finds the battered science ship USS Equinox under attack by creatures from another realm, the two captains must decide whose crew will take the lead. Rudolph Ransom wants to abandon his ship on his own terms, but Janeway quotes the regulation and notes that Voyager is plainly the stronger vessel.
Ransom concedes, remarking that he looked the rule up himself. The exchange sets up the larger conflict of the story, in which Janeway learns that Ransom has been killing the creatures to fuel his engines and has broken far weightier rules than this one.