When a string of the form "%n" (where n is in the range 0 through 9) occurs in the source string, a WM_SUBSTITUTESTRING message is sent to the specified window. This message returns a text string to use as a substitution for "%n" in the destination string, which is otherwise an exact copy of the source string.

If "%%" occurs in the source, "%" is copied to the destination, but no other substitution occurs. If "%x" occurs in the source, where x is not a digit or "%", the source is copied unchanged to the destination. The source and destination strings must not overlap in memory.

This function is particularly useful for displaying variable information in dialogs, menus, and other user-interface calls. Variable information can include such things as file names, which cannot be statically declared within resource files.

This function is called by the system while creating child windows in a dialog box. It allows the child windows to perform textual substitutions in their window text.