The B position for the base umpire is defined as two to three steps to the left, two to three steps to the rear of the second baseman in his normal position. He will be positioned approximately 25 to 30 feet from first base or second base, which will give him a good angle and position to make calls at first or second base. He is only in this position because he has a runner at first base. As he is setting up, his responsibilities are the same as watching the pitcher, helping his partner, but he also has the responsibility of the runner leaving early. So the base umpire does not want to crowd in where he cannot see that runner on first leaving early.
So he will have to adjust so that as he focuses on home plate and is squared up to home plate, that he can see the runner at first base if he happens to leave early. This is the B position.