When you create a new layout in Sitecore CMS, most of the content is standard for an ASPX page, except for this extra line of content: <sc:VisitorIdentification runat="server" />
The name of the tag seems to relate it to Sitecore Analytics since it states that it is meant for something related to "visitor identification". If you follow the content rendering of the page you will find that it generates this tag:
<link href="/layouts/System/VisitorIdentification.aspx" rel="stylesheet" type="text/css" />
As well as content of the style sheet being (/layouts/System/VisitorIdentification.aspx):
Now if you are wondering what the purpose of generating all this content is and decided to search through Sitecore documentation, you will find out that the VisitorIdentification web control’s purpose is to help in the identification of robots, since most will not request style sheets.
[Image courtesy of LongitudeLatitude via Flickr Creative Commons]