What do the schemes login, login_post, admin, and relative mean?

Question

From the Codex

if ( ! $scheme ) {
        $scheme = is_ssl() ? 'https' : 'http';
} elseif ( $scheme === 'admin' || $scheme === 'login' || $scheme === 'login_post' || $scheme === 'rpc' ) {
        $scheme = is_ssl() || force_ssl_admin() ? 'https' : 'http';
} elseif ( $scheme !== 'http' && $scheme !== 'https' && $scheme !== 'relative' ) {
        $scheme = is_ssl() ? 'https' : 'http';
}