AS3 RFC - Unknown FTP location

As I was going through latest AS3 RFC draft. In section 7.3, I found following 
text:

"The ftp-url field is specified as an RFC 1738 


   <URL:ftp://host.com:port/url-path>, and while it MUST be present, 


   it may be ignored if the ftp-url points to an unknown location."


This is little bit subjective! What are criteria for declaring an ftp-url as an 
unknown location? Any thoughts? Should criteria be specified in the RFC?

Thanks an advance!

Surendra