Grid 641 was the Borg spatial designation for a region of space in the Delta Quadrant near the transwarp hub that was located in Grid 986.
In 2378, the Borg Queen sent a Borg cube to covertly follow the USS Voyager in this grid. The cube traveled on a flight path suggesting it hadn't detected the Voyager. (VOY: "Endgame")
The grid designation was depicted on a computer screen in the episode.