ICL (Windows Icon Library File)
The ICL System file format
ICL files are usually found on computers running on the Windows operating system. They are known as icon library files. As the name suggests, ICL files represent a whole collection of icons. They are used to replace an existing set of icons with another one, e.g. when changing a Windows desktop theme.
Technical details of ICL files
ICL files contain a set of icons, grouped together by the library file. When changing a file icon, the ICL file is called to provide the selection of icons in this library to the user. They can then choose an icon from the list to assign it to the shortcut or file they wanted to change. The ICL file can only be called when changing the icon of a shortcut. Programs and other file types can not have an icon from an ICL file assigned.