The Boundary was a portal on the shore of an unnamed planet in the far future, activated by the proximity of humanoid people. It worked as a gateway, a direct route into another random part of the universe. The Boundary patterns were never the same twice, so it was used by the humans during the late Cyber-wars to escape the Cybermen. It was guarded by Ko Sharmus, who helped the fugitives to get through it.
Eventually, it was a gateway to the Gallifrey ravaged by the Master. (TV: Ascension of the Cybermen, The Timeless Children)