There are several differences between the high bar back squat and the low bar back squat. Each type of squat has it’s supporters.
The high bar back squat is often advocated by Olympic weightlifters because the more upright position is closer to that of a front squat or overhead squat. However, the movement does have some disadvantages. A large portion of the weight is sitting on the lifters spine and it does not bring in as much of the posterior chain as a low bar back squat.
A low bar back squat, where the bar sits just below the spine of the scapula, has 3 main differences from the high bar back squat. 1. By placing the weight lower on the back, the lever arm between the load and the hips is shortened, often allowing the athlete to lift more weight. 2. The weight sits on the shoulder muscles (deltoids and infraspinatus) rather than directly on the bones of the spine, 3. The more horizontal position of the back brings more of the posterior chain into the lift. For these reasons, the low bar position is often favored by people who use the movement as part of a strength building program to support CrossFit or other sports. However, as stated above, the more horizontal position in the bottom of the squat does not as directly correlate with the more upright squats of the Olympic lifters in competition.