The combined basePath and relativePath.
The VirtualPathUtility.Combine(string, string) method returns the value of the HttpRuntime.AppDomainAppVirtualPath property if the relative path is exactly a tilde (~).
The VirtualPathUtility.Combine(string, string) method uses the HttpRuntime.AppDomainAppVirtualPath property value rather than the base path, if the relative path starts with a tilde and a slash mark (~/); otherwise, the VirtualPathUtility.Combine(string, string) method uses the base path.