Hi, Thanks for the question.
There are two types of preset in the vertical library XY and UV. Generally speaking I would use XY libraries for simple planes and UV for anything more complex. How you align the pivots depends on which of these you're using. In the image below I've annotated the Z axis only when it's important. If it's not shown, you don't need to worry. XY Presets
These presets work by projecting items along the FP object's local Z
1 - If you are using a spline, the Forest Pack object must be rotated so that it's vertical and the Z axis is towards a plane defined by the spline.
2 - If you are using a surface the forest object and the surface should be aligned so that their Z axes and in concordance. In the case of a flat wall for example, the easiest way to fix the pivot is often to rotate it so that it lays flat on the world X/Y plane, reset the XForm, and then rotate it back into position. UV Presets
These presets are ideal for irregular areas because they use the UVW coordinates of the surface so you won't need to adjust the pivots. You will however need to ensure the UVW on the surface are not distorted and the correct orientation.
UV Presets are not designed to be used with splines.
I hope that helps. I will record a quick tutorial to explain this in more depth.
Please let me know if you have any further questions.