There is no single section that tells you what (X) troops do. It is usually in the troop definitions and the combat results. For example, Bd(X):
Definitions:
Exception (X): Men with weapons unusually dangerous to heavily armoured knights, such as ... . Treated as Blades (F), but destroy Knights scoring less.
Combat outcome:
If an element’s total is less than that of its opponent, but more than half:
Knights. Destroyed by Elephants, Expendables or Blades (X).
So a Kn element losing to (but not doubled by) any other sort of Blade will usually just recoil, but if it loses to a Bd (X), even if the scores differ only one, it's dead. Otherwise a Bd(X) fights as any other Bd - but look at the rear support factors for another exception.