What is a 'Screen Name' in Thunderbird's and SeaMonkey's address books?

[James Michael Fultz]

* Detlef Lechner <Detlef.Lechner at gmx.net> [2009-10-01 15:20 +0200]:
>Hello Liste, 
> 
>What is a 'Screen Name'? For example, in SeaMonkey > Address Book > Address 
>Books=Collected Addresses > <any entry> Properties > (Edit Card for 
><displayname>) > (Internet) > text field 'Screen Name:'. 
>I'd like to point out that there is a 'Nickname:' text field as well. 
> 
>How is an entry in this field being used? 

Screen name refers to an AOL Instant Messenger (AIM) screen name.