The function is discussed in Session Management in the product help, but essentially it's used to allow the Control Room operator to send a Stop signal to a running process. The process will not stop immediately, all the signal does is change the output of the IsStopRequested function to True. What the process does with IsStopRequested is entirely dependent on the design, although if you look at the Template processes we provide, they use a Decision to decide whether to work another queue item or to go to the End of the main page.