Thank you very much for your replies.
IMO, an element compare his grading with the grading of the enemy element in contact if this is the sole contacting enemy element.
If the enemies in contact are two or more, the element compare his grading only with the enemy element in frontal contact.
So, if my LH(S) is contacted on the front by enemy LH(S) and on flank by enemy LH(O), CF is 1(O) vs 2(O).
If my Cv(S) is contacted on the rear by enemy Cv(S), CF is 1(O) vs 3(O). If dice are 3 vs 1, my Cv(S) is not killed !!!