Can anyone help me understand how the Field of Battle was controlled

I am currently reading *The Crusades: The War for the Holy Land* by Steve Tibbles. The book says that the literal battlefield itself was controlled by the Franks and Muslims but I do not understand how and under what circumstances this type of control could take place. Why was it not always the case that the field of battle could be controlled? There seems to be not consensus on this from other books that I have read. A few were very vague and another said that it could be controlled and the control of the field of battle essentially decides who "wins" the war. What I have not been able to find is any kind of consistency in regards to how the field of battle is controlled and under what circumstances.