When RewriteRule directive is used in per-directory configuration files (.htaccess) it will automatically strip the local directory prefix from the path and apply rules only to the remainder. RewriteBase directive allows you to explicitly specify a base for the rules, i.e. the part that will be stripped.
Unlike Apache mod_rewrite, ISAPI_Rewrite has access not only to the physical path, but also to the virtual path and can automatically choose correct base. So this directive is preserved only for the compatibility reasons.
URL-path can be root relative path or empty. Empty
URL-path value meand thet rule base will equal to web site root.