The extension.lcf identifies a “Library Container file”. The default ArchiCAD 17 Library is stored in an.lcf, but you can also create your own Library Container file, or extract its contents, using the File > Libraries and Objects > Create/Extract a Container command.

Since the Library Container file is a single file, yet contains all the objects used in your project, it lets you keep all the objects used in your projects in one place, while keeping hierarchies intact within the .lcf.