Set to Top & Bottom Edge

There are two options for setting the orientation of a residential (i.e. flush-mount) rooftop: "Set to Top", and "Set to Bottom": 

Alignment Options for Residential Array

Why is this different from fixed tilt? Residential roofs are different from commercial roofs in a number of ways - including the fact that you rarely have a perfect rectangle. In some situations, there is no good "front" side of the roof. For example, see the example below: 

In this case, the "front" of the roof is just a short edge on just one side of the bisecting roof (circled in red). These short lines generally don't work well for setting the orientation of the entire plane. Instead, in this case, you would want to use the ridge line (circled in green) to set the azimuth, using "Set to Top". 

Separately, there are also roofs where there is no good ridge line, and you would want to use the gutter (green edge) to set the orientation of the roof, using "Set to Bottom": 

Of course, in simple roofs that are rectangular, both options will work! And if used correctly (Top for ridge, Bottom for gutter), will produce the same azimuth angle.