Duration policy
Sets the Population duration: unlimited, set time, or iterations. One iteration represents one Virtual User played once, through to its end. When iterations are selected, a Population runtime must be defined by entering the number of iterations in the Load Variation Policy panel.
In any event, a test can be stopped at any point by clicking on the 
Stop playing the performance test button.
A single click tells each Population to execute the stop policy set for it, then stops the test. If the test continues running, a second click stops the test immediately.