Above will place the dimension text above the dimension line. It primarily affects horizontal, radial and diameter dimensions, leaving vertical and angular dimensions still centered. Outside pretty much affects all dimensions. This option will place the dimension text on the side of the dimension line that is the farthest away from what you're dimensioning. The easiest way to clearly understand the different settings is to select each and watch the dimension image change.
Center positions the dimension text centered between the dimension lines.
It doesn't appear that either one of the last two options affects leaders or radial dimensions. They do affect angular dimensions, even though the image doesn't indicate this fact. When dimupt is on, you manually control the horizontal placement yourself when placing the dimensions on your drawing. This setting can also be found in the lower left-hand corner of the FIT tab under Place Text Manually When Dimensioning. If this setting is checked, AutoCAD ignores the horizontal justification settings.