An interplanetary code was a code in which communications were transmitted between planets.
In 2267, Nyota Uhura recognized the as-yet-unidentified Nomad's transmissions as being in an old-style interplanetary code rather than Starfleet code, which Spock found fascinating. (TOS: "The Changeling")
In 2366, USS Enterprise-D picked up a transmission from Orelious IX that was in an ancient interplanetary code. (TNG: "Booby Trap")