Ultima Online: Renaissance
Ultima Online: Renaissance is the second expansion for UO. Released on April 3, 2000, it added content, fixed bugs, and made several gameplay changes. It introduced the Trammel facet. Although Factions didn't make in for release, it was added a couple months later.
UO:R (as this expansion is abbreviated to) basically "mirrored" the UO map.
The "mirror" world became the Facet of Trammel, and had a rules set that limited Player vs. Player PvP combat.
The "old world" remained, but was now called Felucca.
Confusingly, Trammel and Felucca are also the names of Sosaria's twin moons.
Politically, Trammel was under the jurisdiction first of Lord British and then the Ruling Council. Felucca was the subject of a never-ending civil war that was introduced a few months after UO:R's release (see Factions).
Since the release of UO:R to this date, the amount of players who actually visit Felucca is (according to official statements of the UO team) a very small (if vocal) percentage of the player base.
For more on the various rules set differences between, and history/fiction of Felucca and Trammel, see the each facet's individual articles.