This File menu command closes the current document window. You’ll be prompted to save any changes you’ve made since you last saved. The Close command does the same thing as clicking the Close box in the upper-left (Mac) or upper-right (Windows) corner of the window’s title bar. Close does not quit Sketchpad itself.
The keyboard shortcut for Close is Ctrl+W (Windows) or W (Mac). You can also use Ctrl+F4 in Windows.