Here is a pictorial explanation that may help.
On the left is a natural humerus, see how the bone extends beyond the end of the shoulder?
At the center is a total replacement, where there is an intact rotator cuff to anchor and stabilize the implant. The implant mimics the natural arm - ball on the humerus, socket in the shoulder. It retains a more natural shape.
If there is no rotator cuff available, as in it is disintegrated or completely torn, the ball and socket would be unstable, resulting in dislocations. So someone figured out how to adapt the implant so that the deltoid muscle, across the back of the shoulder, can stabilize the implant. To do this, a modified ball is inserted into the shoulder socket, and a modified socket is created in the humerus.
The image on the right shows this. You can see it changes the position of the arm and shoulder substantially, with a more rounded form and less width. In addition, you probably will have less range of motion that with a full replacement.

Those changes on usage and appearance are what prompted me to try a rotator cuff repair instead (with the caveat that it might not work.) Nine months later, it is going okay and I am still being careful because the full healing and strengthening time is between one and two years for an extensive repair.

PS Connect reorganized the photos - left is full replacement, center is before, and right is reverse replacement