If a formation includes both war engines and non-war engine
units then an attacker must state whether any attacks he makes
on the formation will be directed at the war engines or the
other units in the formation. Attacks directed at the war
engines can only be allocated against war engines if they hit,
while attacks directed at other units may not be allocated to the
war engines in the formation.

Well consider a mixed formation of WE and AV....both a vaild AT targets but you have to chose which to target.