Ever wonder what the difference was between selecting Rafter or Truss when doing a Roof by Footprint and utilizing Pick Walls? (Notice, I mentioned Roof by Footprint and Pick Walls...you have to use these tools/options to get the selection of Rafter or Truss.) Well, the difference is quite simple but can be confusing to some when laying out the footprint lines.
When Selecting the Rafter Option, the slope starts at the INTERIOR face of the wall. If you use the Truss Option, the slope will start at the EXTERIOR face of the wall. The confusing part for some is that the sketch line doesn't change when changing the Rafter or Truss parameter. As you can see in the images, the overhang doesn't change but the "base point" of the wall does change height. Notice how the dimension from the level to the top of the fascia line...the roof is moving vertically to adjust where the slope point starts. So the sketch line doesn't change location to change where the slope starts, the Rafter or Truss option moves the roof up or down to change where the slope starts.
EDIT: Forgot to mention one little thing...using Extend to Wall Core also works the same way. The above images used the face of the wall for the overhang, not the face of the core. If you check the box in the options bar to Extend to Wall Core, the slope point will be from the core face. This would give a more realistic representation of the actual construction for most stud conditions, where the slope point would be in-line with the stud face and not a finish face.