[Therion] Additional morphing points (type "extra") explained

Hello, i struggle to understand how the extra morphing points are meant to be used. How do they work technically? How do i correctly set them? Are they linked to the nearest station and could i use more than two extra points? Are they linked to LRUD data? I already investigated the "morohing" example from the documentation (both in code and in thbook), so i think it would be good to stick to this example. Thank you very much!