The treaty as mentioned on a map
The Treaty of Ka'Tann was a diplomatic agreement negotiated by Ambassador V'Lar during the late 21st or early 22nd century.
While T'Pol was in her early schooling on Vulcan, she traveled to Vulcana Regar to see V'Lar, who had just negotiated the Treaty of Ka'Tann. At one of the recesses during the second Ka'Tann Conference, T'Pol approached V'Lar to question her negotiating tactics, which V'Lar later recalled as being "quite presumptuous coming from one so young." However, she would also note that T'Pol's bluntness made her reconsider some of her positions. (ENT: "Fallen Hero")
As of the early 24th century, the Treaty of Ka'Tann prohibited Starfleet from crossing certain segments of the Federation border, for example in the vicinity of the Talarian Republic. (Star Trek: Section 31)