Ultima Online is part of the main Ultima games
As a series of pocket dimensions, each shard reflects a potential future development of Britannia had it not been tampered with by the Guardian and destroyed during Ascension. These shards exist as gems within the main games, as many gems can be found by the Avatar. The Time Lord and Sherry from the main games have visited the shards, as detailed in books. All the shards are destroyed when the Avatar and the Guardian recreate the world as Alucinor.Ultima Online 2 is an alternate future Britannia
In Ultima IX, the Avatar has a copy of Ultima Online 2, representing the alternate future where Lord Blackthorne develops steampunk technology, recounted as the Technocrat War. This alternate future becomes reflected in the events of "Lord Blackthorn's Revenge," though it does not come to full fruition. The alternate future is destroyed when Britannia is recreated as Alucinor.