Earth-123 is the Earth in every Dimension.
In Dimension 11, Ben was the only one to receive the Omnitrix but instead of saving people from bad guys, he became a bad guy.
In this Dimension, the Omnitrix was never sent to Earth. Azmuth, the creator decided to keep it for himself and use it for its intended purpose.
In this Dimension, Ben Tennyson accidentally triggered the self destruct mode for the Omnitrix and the whole universe was destroyed because he could not find the creator of the Omnitrix.