CHAT was developed by cognitive psychologists. They were mostly interested in how understanding of the real world is developed, and also how meaning is drawn from what one understands, and one's ability to learn and respond from what one have learnt.
To learn, helps one to form a "mental model", and this mental model must help one to think about the real world, to understand about the real world and to fit into the real world. One should be able to develop new insight and meaning by forming a "mental model". The formation of a "mental model" is a cognitive process that is based on experiences and understanding.
Roth, W.M.& Lee, Y. 2007. Vygotsky's Neglected Legacy: Cultural -Historical Activity Theory. Review of Educational Research. June 2007, Vol.77, No2, pp 186-232.