they both adjust. but on my fork, half is spring, half is air. so it is very versatile but on my back. i honestly have no idea how to adjust it! all i know is, it doesn't bottom out when i bunny hop on it, and it is soft so that is all that matters for me.
well, whatever makes you happy! BTW I think coil shocks like that should have preload adjustment (turn the spring's retaining nut) and rebound adjustment. When we meet I can see if I can help you adjust it. I spent a lot of time in the Highland bike shop and I learned some stuff about maintenance.
Lansdale, Pennsylvania, United States 
