optional stop

基本解释

Have a "Stop Music" button. If you are a musician and have music on your site, make listening to the music optional.
网站设置“停止”按钮，如果你擅长音乐创作，并且在网站中设置音乐，你可以让访问者选择性聆听。
Controlling room: it is made up of the optional push button, the sudden switch, the alarm bell push button and the electric lock, controlling up, down and stop of the lift car;
操纵厢：由层楼选择按钮、急停开关、警铃按钮、电气锁组成；控制轿厢的上行、下行和停止。
wuxifanyi.com
The illustration of optional zone and dilemma zone is given: when a driver's distance from stop line is longer than the minimum distance that is required for a driver to stop, he is at optional zone.
举例分析了当驾驶员离停车线的距离大于其停车所需最小距离时，他处于可选择区域，反之，则处于困境区域。