Those who have the X-Antibody are generally distinguished by the X in their name and sometimes an X placed on their body somewhere or a gem (example: Dorumon's on his forehead, WarGreymon X's on the front portion of his front horn). There are few with the X-Antibody that aren't distinguished by an X after their name because there is no other version of them in existence.
The X Digimon fled to a new Digital World which was split into three Terminals (the three terminals are named after the Norns from Norse Mythology) consisting of Urd, a past plain which is a volcanic wasteland, inhabited by dinosaur and dragon Digimon, Verdani (mispronounced as Versandi) is the present which is a world of lush greenery and is home to beast, bird, plant and other nature Digimon, and Skuld is the future, a high-tech city where machine and insect Digimon dwell. Yggdrasil sent the Royal Knights to eliminate the X Digimon.
Also, in America, Digimon with the X-Antibody are prominently featured in the card series Digimon: Operation X Certain Digimon have the X Antibody, some Digimon are vulnerable to these Digimon, and another group of Digimon can easily destroy Digimon with the X-Antibody. Modify cards can also add, destroy, alter, or get power from the X Antibody.