A secondary window can be modeless or modal. A modeless secondary window allows the user to interact with either the secondary window or the primary window, just as the user can switch between primary windows. It is also well suited to situations where the user wants to repeat an action — for example, finding the occurrence of a word or formatting the properties of text.
A modal secondary window requires the user to complete interaction within the secondary window and close it before continuing with any further interaction outside the window. A secondary window can be modal in respect to its primary window or the system. In the latter case, the user must respond and close the window before interacting with any other windows or applications.
Because modal secondary windows restrict the user's choice, use them sparingly. Limit their use to situations when additional information is required to complete a command or when it is important to prevent any further interaction until satisfying a condition. Avoid using system modal secondary windows unless your application operates as a system level utility and then only use them in severe situations — for example, when an impending fatal system error or unrecoverable condition occurs.