x1 (du'u: proposition or kau-question) cannot be assigned a definite truth-value or answer
Brivla form of na'i. i.e. x1 is something one would answer na'i to. Useful sumtcita places include ni'i for the logical reasoning for x1 being na'itfa, and va'o for the conditions / contexts where x1 is na'itfa. See jo'atnu