I've discovered a strange behavior of altitude range. It seems it works in object's local space rather than the world space. Since ForestPack is mainly landscaping tool, i think altitude range should be expressed in world space, not local space. Most of the times you are building environments using multiple forest objects, so Altitude range should remain consistent across them.
Especially when the way ForestPack objects are created is done by clicking on the existing object to be used as a surface, instead of placing ForestPack helper into the scene. This will automatically align the local space of ForestPack helper to the distribution surface, and therefore adopt it's coordinates. As a result, if you create two different Forest objects on two different surfaces at different Z axis coordinates, setting Top and Bottom altitude will have very different meaning for each of them. That's not exactly intuitive.