What does the mce_src image tag do?

The mce_src tag is used to link to an image at a location on the web.  The text below came from here: http://www.howto-outlook.com/howto/signatures.htm#company_logo.

Creating an HTML signature with a picture/company logo
When you want to add a picture or (company) logo to your signature youll have to use the advanced signature editor. This will only create a HTML signature. This is no problem as a Plain text signature cant hold embedded pictures and Rich Text is not a recommended format to send outside your local company as it is unsupported by many other mailclients and/or servers. For more info regarding Rich Text click here.

A thing that is important is that you link correctly to the file. By default when using Word or Front Page as the advanced editor they will place a copy of the picture in the Signatures folder when the picture gets inserted by Insert-> Picture& By modifying the HTML Source code you can also link to a picture on a web location. By default Outlook will retrieve the picture from the Internet before sending the message and embed it in-line.

The code would be looking like this;
<img src="http://www.domain.com/pictures/picture.jpg" mce_src="http://www.domain.com/pictures/picture.jpg">

Thanks. I understand all that. What I wanted to know is who's tag is this. It isn't in the HTML standard, so someone must have come up with it, and if so, what did they define as its usage.

The link provided by w8web is the same as I provided, which prompted my question. I know how to do the signature, I would like to know why. I have been unable to find an explanation of the mce_src tag anywhere.

Oh well, sounds like just another one of those undocumented MS "features".

Thanks