I'm not sure what you mean by "subrule". Just to clarify the terminology, there are "Rules" which contain one or more "Conditions". The rules are evaluated from top to bottom in the list. The first rule that has ALL of it's "conditions" pass will be used for the transition. Only one rule will be used, so once a rule has all it's conditions pass, then no rules below it will be checked.
Both the scene that is going online, and the one that is already online (if there is one), will evaluate the rules within their own scenes. The scene going online needs to choose an IN effect, while the one that is already online needs to choose an OUT effect.
If you go to Tools->Debug->LiveLog you when you take something online, you can see which Rule it ends up using for the transition.
#XPression