الوقت ظرفاً للفعل كالصلاة.
In prayer, time functions as a container for performance.
The four obligatory units of ẓuhr prayer do not occupy the entire ẓuhr period. Other lawful prayers or acts may also occur within that window.
Therefore, the obligation of one prayer within the period does not make every other prayer within it impossible or transform a voluntary prayer into that particular obligation.
This yields the first important consequence of a container-type period: since several prayer acts remain possible in one period, each act may need an intention distinguishing which prayer it is.