Jetsün Chokyi Gyaltsen:
Definition of the nature-truth body: A final sphere endowed with the two purities.
The nature-truth body being the natural purity.
The nature-truth body being the adventitious purity.
The natural purity is the absence of inherent existence of the enlightened mind, and the adventitious purity is the abandonment of the two obscurations attained through the power of the antidote.
The emptiness of a mind with obscurations becomes the nature-truth body with the attainment of enlightenment, but it already possesses the natural purity of the absence of inherent existence. (Translated this means the sky is the limit)