It's Big OR Tall. There are three sizing conventions - Regular, Big, and Tall. The L is kind of optional and doesn't mean much, except you almost always see it in talls. So 5X, 5XB, and 5XLT are three different sizes.
What does 4XB mean?
the 4xb is standard 4x sleeve length. but with an average of 3-5 inches in the chest area and average of 3 inches in neck area. when compared to 4xlt.