The four levels (wei, qi, ying, xue) wèi qì yíng xuè · 卫气营血辨证

A framework from the warm-disease school that describes the depth of an acute febrile illness in four levels: wei (defence, at the surface), qi (the active stage with marked fever), ying (nutritive, with deeper disturbance), and xue (Blood, the deepest, with bleeding or rash).

It is essentially a severity ladder, and it was built to reason about epidemics. That is why it reads more like a staging system than the static patterns used elsewhere in TCM.