In the Vedas the supramental link-plane between the divine and the lower hemispheres is called mahas. In harmony with for example the Taittiriya Upaniṣad, Sri Aurobindo often calls it the vijñānamaya kośa. In later Sanskrit and in Hindi, vijñāna is used for the much lower faculty of the intellect, but in this text the word vijñāna is always used in te way Sri Aurobindo uses it: for the link-plane which has a type of consciousness that is far above even the highest levels of mental consciousness.
The differences between the two are described in more detail at the end of the chapter on The Evolution of Consciousness [ADD LINK] and in the relevant sections of the chapters on the Structure of the Personality and Knowledge[ADD LINKS].