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

[Detlef Lechner]

Avi Greenbury wrote:

> Detlef Lechner <Detlef.Lechner at gmx.net> wrote:

>> I still do not understand what this has to do with the free E-Mail
>> programs Thunderbird and SeaMonkeys address book.
>>
> 
> It's an attribute of a person that one might want to store in the
> address book. Same reason there are (probably) fields for postal
> address and fax numbers.

It is obviously more than that:
https://wiki.mozilla.org/Thunderbird:Help_Documentation:Using_the_Address_Book#Contact_Tab
speaks about an implemented IM functionality.

Regards,
Detlef Lechner