Understanding Slope, Hip, and Gable
In addition to the manual

What is Slope / Pitch
The slope of a roof refers to the angle the roof line runs compared to the horizon. In 3DHA the default slope (or Pitch) is 6-in-12. This value can be changed to fractions such as 4-1/2 in 12 or 5-1/4 in 12. The numbers refer to the distance the roof will RISE compared to the distance that it RUNs along the horizon. So for a 6 in 12 roof, that would mean the roof Rises 6 inches for every 12 inches you move along the horizon. (in cross section)
Various Pitches
Some very common pitches would be 3-in-12, 6-in-12 (shown above), and 12-in-12. The drawing to the right shows the general difference from one pitch to another.
How to Set the Pitch
In Plan view, click on the Roof Tool Bar and the default tool on that roof tool bar is the Roof at Wall tool. (v3.0)

With this tool active, you can simply click on the wall you want to change and the Roof Section at Wall dialog box will open. The default type of roof in 3DHA is a hip roof so no check box requires attention. Here you will just fill out the Pitch text box. Completely erase the 6(D) and replace it with the value you prefer. If you don't erase the (D), the Pitch will revert back to the Default value.
Hip vs. Gable Roof Section
When viewed in a cross section, a Hip roof has a slope running from the top plate of the wall UP towards the ridge of the roof. A Gable wall runs past the top plate, all the way to the roof, turning 90 degrees when it meets the roof. In the drawing to the right you are looking at the Gabled End of the house on the left, but seeing the angle of the Hip roof on the right side. The drawing on the right has you looking at the house from a side that has a Hip roof, but illustrating the Gabled end and how it is at a 90 degree angle to the wall. (No Slope)
more on....Hip vs. Gable
Notice that the Pitch text box is disabled (grayed-out) when the Roof style is changed to Full Gable Wall. (lower left) That is because a Gable end wall has no Pitch or slope running up from the top plate to the ridge.
The distinction between a Hip wall and a Gabled wall is very important if you plan to make 3DHA produce the roof you expect. 3DHA will create a single roof style if you select that roof style in the Make Roof dialog box, but 3DHA will change the roof style of individual walls if you override them in the Roof at Wall dialog box of different walls.