The chapter on the nine variations has a different grain from what came before. Until now Sun Tzu set principles. Here he speaks of when those principles have to give way.
A battlefield has moments no textbook can settle in advance.
途有所不由，軍有所不擊，城有所不攻，地有所不爭，君命有所不受
tu you suo bu you, jun you suo bu ji, cheng you suo bu gong, di you suo bu zheng, jun ming you suo bu shou
There are roads you do not travel, armies you do not strike, cities you do not attack, ground you do not contend for, and orders from the ruler you do not accept. (The Art of War, Nine Variations, translated directly)