It’s from the Census. It’ll be through your data management and registration system eg SIMS, Integris etc. The codes are N, no special support, K SEN support, and E, EHCP. But you’re right they don’t really fit with a graduated response.
This is how Ofsted capture data about the number of SEN pupils. Your internal data can show more if a differentiated approach – as long as someone is on the register somewhere as a K, it doesn’t matter if you routinely describe them as a T! It’s just about systems