IRC local alias vs screen name

Seting up an IRC account in Pidgin I came accross input fields for "screen name" and "local alias" and got confused about the purpose of the latter, so I set different names for each one and proceeded to chat away hoping to stumble with some clue.

Enlightment came when I /me that I was leaving the channel; instead of showing my screen name the output of the /me command displayed my local alias. I restarted Pidgin right away and I then my local alias was displayed in the chat room user list instead of my screen name. I /whois my local name but the command will only respond if I /whois my screen name, even if in my IRC client I see myself listed featured as my local alias.

So, I guess having a local alias option is useful if instead of my screen name I want to see myself listed just as for example "me" while others in the room continue to see my screen name.

Now I wonder what's the purpose of the "user name" option, which value shows up as part of a /whois command, appended to my DNS resolved IP address.