The tribunal is based in the Netherlands but applies Lebanese law. However, Italian Judge Antonio Cassese said it should use a "more extensive" definition of the key crime under its jurisdiction - terrorism - than the one used in Lebanese courts.
The ruling is unlikely to affect any trial of Hariri's alleged assassins, who killed the prime minister and 22 others with a massive truck bomb on Beirut's Mediterranean seafront on Feb. 14, 2005. However, it could come into play if prosecutors go on to indict suspects in other terror attacks related to the Hariri case.
The Special Tribunal for Lebanon is the first court set up specifically to prosecute acts of terrorism, and its jurisprudence will be carefully studied by legal scholars and diplomats who have spent years trying to pin down a definition of terrorism as an international crime.
Cassese pointed out that a U.N. committee remains deadlocked after 14 years of attempting to draft a terrorism convention.
Lebanese judges "tend to place a narrow interpretation" on methods that can be used to carry out a terrorist attack, Cassese said, including "explosive devices, inflammable materials, poisonous or incendiary products, or infectious or microbial agents."
Cassese and four other appeals judges unanimously ruled the tribunal was justified in using a definition of terrorism "more extensive" than the one used in Lebanese courts.
That could include the use of weapons such as hand guns and machine guns, he said.
Cassese said the 152-page ruling should "help the tribunal take a firm, steady and rapid course in its pursuit of justice."
Prosecutors submitted a sealed indictment on Jan. 17 to a pretrial judge who must decide whether the supporting evidence merits a trial. The judge's review is expected to take at least six to 10 weeks and the indictment's contents will only be revealed if he confirms the document.
It is widely expected that members of the Shiite militant group Hezbollah will be named in the indictment. Hezbollah denies any involvement in Hariri's slaying, and its leader, Sheik Hassan Nasrallah, has said his group would "cut off the hand" of anyone who tries to arrest any of its members.
Unlike other international courts, the Hariri tribunal can hold trials in absentia if suspects cannot be arrested.