From Digimon Masters Online Wiki - DMO Wiki
X-Antibody Factor can be used to unlock X-Antibody Digimon Evolution and increase the powers of Digimon. To use the digimon in the form X-Antibody, you must be have a XAI (X-Antibody Indicator) equipped in your Tamer window, X-Evolution form comsumes certain Tamer’s X-Gauge (like a Tamer D.S. bar) to evolve and continually consumed after Digivolution, X-Gauge can be restored by using X-Antibody item. Using the digimon in the X-Antibody form, the digimon will have a purple aura, clearly presenting the difference to their original counterpart.
X-Antibody Digimon Available