• Archive file name
• HTTP URL of an archive file name. This URL may contain a star (*) for the version. In this case pack_install asks for the directory content and selects the latest version.
• GIT URL (not well supported yet)
• A local directory name given as file:// URL or '.'
• A package name. This queries the package repository at http://www.swi-prolog.org

After resolving the type of package, pack_install/2 is used to do the actual installation.