A property sheet can have only one active page at a time. The page that has the activation is at the foreground of the overlapping stack of pages. The user activates a page by selecting its tab; an application activates a page by using the PSM_SETCURSEL message.
The property sheet sends the PSN_KILLACTIVE notification message to the page that is about to lose the activation. In response, the page should validate any changes that the user has made to the page. If the page requires additional user input before losing the activation, it should use the SetWindowLong function to set the DWL_MSGRESULT value of the page to TRUE. Also, the page should display a message box that describes the problem and provides the recommended action. The page should set DWL_MSGRESULT to FALSE when it is okay to lose the activation.
Before the page that is gaining the activation is visible, the property sheet sends the PSN_SETACTIVE notification message to the page. The page should respond by initializing its control windows.