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Full Idea
According to the Principle of Bipolarity, every meaningful sentence must be capable both of being true and of being false. It is not enough merely that every sentence must be either true or false (which is Bivalence).
Gist of Idea
Bipolarity adds to Bivalence the capacity for both truth values
Source
Michael Morris (Guidebook to Wittgenstein's Tractatus [2008], 3D)
Book Ref
Morris,Michael: 'Guidebook to Wittgenstein's Tractatus' [Routledge 2008], p.133
A Reaction
It is said that early Wittgenstein endorses this. That is, in addition to being true, the sentence must be capable of falsehood (and vice versa). This seems to be flirting with the verification principle. I presume it is 'affirmative' sentences.