It’s part of the original ID that Bricks gives the element. Also, it’s the data-id attribute given to elements in the builder if you inspect with dev tools.
@zippa71 Did you find a way to find the element, when you know the id? I would love to do the same thing. I must have placed custom CSS on an element in the wrong place, and I cannot find in bricks where it is… but I can find the id in chrome dev tools.