AB is short for Aurora Borealis. This refers to the solid color diamond drills with a beautiful Aurora Borealis rainbow coat that gives it an opalescent shine on the surface. The appearance can differ, it can be a shiny soft pearl-like coating to a strong dual-color reflection.
They are used to highlight parts of the diamond painting pattern adding dazzling and mesmerizing effects to the art, just like the northern light it got its name from.