According to Mou (1989), for Tiantai 天台, function is not the function of the mind as thusness 眞如心, but the mutual creation of all phenomena contained within the trichiliocosm. Thus, the nature 性 of the trichiliocosm resides in the essence 體, while the function 用 of the trichiliocosm resides in cultivation 修. In Zhiliʼs 知禮 Shi bu’er men zhiyao chao 十不二門指要鈔 (Jottings on the Essential Points of the Gates of Ten Non-Dualities), this also applies to the dyadic pairs of matter and mind 色心, internal and external 內外 cultivation and realization 修證, cause and effect 因果, defilement and purity 染淨, environmental and direct rewards 依正, self and other 自他, the three modes of karma 三業 (i.e., body, speech, and mind), provisional and real 權實, and the action and capability of fertilizing (i.e. receiver and giver) 受潤.

References:

Mou, Zongsan 牟宗三. 1989. Bore yu foxing 般若與佛性 . Taiwan:  Taiwan Xuesheng shuju.