I will try to put it as layman as possible... XB is short for 'Cross Back' or simply 'X-back'..
This term is normally used for Gold Asian Arowana and it is to state the level of gold shine of the Gold arowana. If you can see pictures of Gold Asian Aros, you will notice that they have much darker tone on the head region and their back region...normally, for a gold juvenile fish, the dark tone is more prominent. As the fish grows, the gold color of the scales will climb higher up to the back....and eventually, the whole fish would be covered with the gold reaching the high of the back. Thus the term ' Cross Back' or XB for short.