Icon library (.icl) is a format used on Microsoft Windows for storing multiple icons in one file. It is an NE (or maybe PE) "executable" file stripped down to the bare minimum needed to use it as an icon container. The individual icons are resources within the library file, in ICO format.
The .icl files we've examined are NE files that contain icon resources, and that have nearly all the nonessential fields set to 0. One might expect the files to be labeled internally as DLLs, but that is not the case for some or all .icl files. They're just generic NE files.
The origins of the .icl file extension, and any other differences it may have from a typical resource-only .dll file, are unclear.