Well, a quick look at past CEE whitepapers (at cee.mitre.org) will
give you the answer. In general, CEE discussions back in 2005 started
from our collective experience with inadequacy of tree based
taxonomies (category -> event type -> specific event). Over time, they
revolt and kill their creators :-)
It is possible to map an O-A-S CEE taxonomy to any tree-based
taxonomy, of course.