What's URD in iptables?

I've been playing around with various iptables configurations lately, and I noticed that adding a rule which involves --dport 465 (usually reserved for smtps) results in the human-readable dpt:urd showing up when listing rules with iptables -L.

I've never heard of this protocol, and an (admittedly superficial) web search doesn't reveal much about it.

What's URD, and why does iptables show this instead of the (arguably more well-known, or at least better-documented) smtps?

[me@risby ~]$ grep urd /etc/services
urd             465/tcp         smtps   # URL Rendesvous Directory for SSM / SMTP over SSL (TLS)

This CISCO documentation suggests that

URD is the designated transition solution for content providers that want to use not (yet) SSM enabled receiver applications. URD allows to enable existing applications to be SSM capable without modifying any receiver host software (application nor operating system) - as long as the application is started via a web browser. URD stands for "URL Rendezvous Directory" and relies on the last-hop router towards a SSM receiver host to intercept URLs from a webserver to detect the source address of the SSM channel.

and moreover that

Source Specific Multicast (SSM) is an extension of IP Multicast in which datagram traffic is forwarded to receivers from only those multicast sources for which the receivers have explicitly expressed interest.

This is (one reason) why you should always, always use iptables -L -n -v for listing rules!