Obviously if none of the opponents is sufficiently efficient in the prior ranges the fight would move to the next range.
What does the whole idea means. Well the Chain of Defense is a concept of the chain of possible events of possibilities of reach of an opponent's pressure points first or preventing him from reaching your pressure points at various ranges from the longest range to the shortest range.
Most of these concepts are virtual meaning you do not see them but you figure them out with a formula. An attacker is running to your direction. As he get closer, he can change his last step into a kicking lunge or a punching lunge. He can also Jump on your upper body or duck and jump on your lower body. So this can turn into initial impact on your head, torso, arms or legs with whole body weight on a large area of impact or small area of impact depending if he is trying to embrace your limbs, or kick them or punch them or elbow them or knee them.
If you are aware of all these possibilities by observing many past attempts to reach one pressure points in the fighting sports arena, you get a pretty good idea of what they are.
If you are certain that you have devised sufficient possibilities of attacks and counter attacks in these aforementioned ranges of the chain of defense, then you have more options.
The bottom line is you want to attack at the range your are comfortable with, but you need to be able to foil your opponent's attack in all the ranges. In self defense you have less choice to pick the range because you have various scenarios that materialize to you become aware of them in different ranges.
It's all about reaching your opponent's pressure points first understanding which pressure points and the gain of control through them. It is the gain of control of your own safety!
Physical and mental limitations also take part in the decision of how to proceed in your training and in your bad situation scenarios handling. When did you become aware of a serious intent to harm you meaning at what range of the chain of defense? Then ask yourself what are the limitation of the human reaction time in that range if there are any? Then are you going to try to reach his pressure points first meaning preemptively attacking him, or do you not have enough time and you will concentrate your efforts on deflection, escapes and counter attacks...