Sector 902 (also known as Space Sector 902) was a region of space, a sector located somewhere in the space of the galaxy's Alpha or Beta Quadrants. In the 24th century, this remote region was well beyond the edge of the Federation's outer boundary.
History and specifics
Unbeknownst to Captain Jean-Luc Picard and the rest of the Enterprise crew, Starfleet was aware the secretive mission directions had been programmed into Lieutenant Commander Data by Noonien Soong, to be activated only when the ship reached Space Sector 902.
Once the directions were triggered by the ship's entry into Space Sector 902, Data overrode Picard's course orders, and Admiral Thivov sent a brief long-range communication confirming Data's correct course entry.
When the crew was able to unlock the secret of leaving Faltos from Data's secret programs, the ship found itself transported back to Space Sector 902, with no record of how to access the course to Faltos. (TNG comic: "Here Today")