The -a and -b options cause the archive contents to be mounted after or before the mount point's existing contents, in a union dirctory (see bind(1) for details). Normally the archive contents replace the contents of mountpoint. The -s option causes the rest of the archive to be skipped once a file is found in the archive which does not start with any of the given prefixes. This is useful an archive is known to start with a description of its contents (eg, the /wrap directory provided by create(8)), and that is all that is needed under mountpoint.