Use this property if you want to show a top-level window, but don't want to interrupt a user's work by taking the input focus away from the current window. This can be an informational pop-up window or a floating window, such as a Tools palette in the Paint application.
Because this property is read-only, you can only change its value by overriding ShowWithoutActivation in your own form, and programming it to return true.
If your non-activated window needs to use UI controls, you should consider using the ToolStrip controls, such as ToolStripDropDown. These controls are windowless, and will not cause a window to activate when they are selected.