According to the Tenth Doctor, the Tandocca Scale was a "series of wavelengths used as a carrier signal by Migrant Bees" that helped explain why the bees had disappeared from early 21st century Earth. He went on to tell Donna Noble that the wavelengths were "infinitely small… like looking for a speck of cinnamon in the Sahara." The Doctor's discovery of the Tandocca Scale allowed him to see a "Tandocca trail"—a path by which Earth and 26 other celestial bodies had been moved by Davros and the Daleks into the Medusa Cascade. He was then able to use that trail to pilot the TARDIS back to the "stolen" Earth. (TV: The Stolen Earth)
Behind the scenes
Though never seen in the episode, the spelling of Tandocca Scale is used in the online script of The Stolen Earth.