The smallest element of the upper bound is called the minimum upper bound; The largest element of the lower bound is called the maximum lower bound;

Just like in this picture, if you want the minimum upper bound of B and D, you must first find the upper bound of B and D. The point of the upper bound of B and D is only F, and the minimum element of the upper bound can only be F;

If the maximum lower bound of D and E is found, the lower bound of D and E has A, B, c b and C, and then the maximum element of A, B and C is found. Because A, B and C have no maximum element, there is no maximum lower bound.