Officially, the most generic/nonspecific of NU is "su'u"; but people seem to use "nu" more often for the purpose of general abstraction. Personally, I wouldn't find it particularly odd if someone use "nu" for a terbri which the gimste defines as "du'u" or other specific types of abstraction. For example:
mi jinvi lo du'u broda (I think that the proposition "broda" is true) mi jinvi lo nu broda (I think that the event "broda" is true) "jinvi"s x2 is officially to take "du'u". Is "nu" for such objects of mental activity / logical operation discouraged? If so, why? mu'o mi'e tijlan