The screw allows 20% adjustement on trip and is only suppose to be adjusted by factory at time of its manufacture and would effect thermal and magnetic setting to change concurently from what i remember. I will see if i can get you some printed lititure on it.

It is supposed to be factory sealed or otherwise potted so consumer can not easily change it.
problem is that depending on age of breaker the 20% could be a serious factor on a worn breaker already being pushed to its lifes end i somebody starts messing with it you could cause internal damage to the breaker and it will not trip seen this happen a couple of times.
I never liked the fact the screw was visable on the older breakers. I do not think its there on the newer ones.