what are and how can i use DominanceGroups?

like the title, what are and how can i use DominanceGroups?
i know that i can create a dominance group by the NxOgre scene and also that i can associate a body or actor to a dominance group putting the string "dominancegroup: 0" in the actor params, but maybe i am missing something...

or

Or do you want me to copy and paste the page in?

DominanceGroups allow you to make particular actors 'dominate'.

An example might be:
If you had a "projectile" and wanted it to knock over some "boxes", but not have the projectile" be effected by those boxes then you would make "projectile" the dominate actor.

Aside from setting have set the actors body dominace group you need to set
NxScene::setDominanceGroupPair

ok. thanks for the replies...i wanted just a practical example...thanks also for the suggestion of the physX doc...i forgot to saw there...