rockhound is correct.
Cuz infantry directive is different from commandos, as also they
tactics and war strategy...etc. it may even go all the way
different in ops, intelligences, manpower, transport, logistics,
etc...
so inf. officer lets say major or half-colonel normally dun cross
over to go command a CDO battllion, cuz he may not know ways of
battles pertaining to CDO style.
While CDO offr, can always "down-grade" themselves and do
conventional warfare. 
