Classically a truth value is asserted of a proposition: thus by its very nature it must be describable.
Jain Logic has a 'truth value' which is asserted to be true and indescribable; by the above it cannot be asserted of classical propositions.
So, what is it asserted of? I suppose, I'm asking what is the form of a Jain proposition.
Interestingly, assuming Kants metaphysics, one can say that the noumenon is (ontologically) true, in that it exists; but by Kants description, it is indescribable ie undifferentiated.