Please list your source
its from Uworld step 2 (sadly I didnt get the qustion ID for them and the cards are not taged) but if you check the images in the extra section of the cards there are tables from Uworld that classify asthma like this and their corresponding initial treatment level/stage
i went and checked Amboss (since i dont remember the questions ID) it is also mentioned in Amboss , in the asthma article under Assessment of severity
heres the link for it https://next.amboss.com/us/article/Ph0WVf#531iQg0