What does the user icon with the little red up arrow in AD mean?

When looking at the members of a group in AD there are normal members (singe head icon) groups (two heads icon) and then Anonymous Logon / Authenticated Users and Everyone which share an icon which is a single head with a red up arrow.

What does this icon mean?

The icon in question is the user icon but it has a little red up arrow coming out of the user. It's icon number 34 inside dsuiext.dll:

XP/2003

Vista/7

Thanks for any help

Solution 1:

The red up arrow indicates a Foreign Security Principal (FSP). This can be used to represent well known FSPs such as Anonymous Logon or for users in an external domain.

Technet: How Security Principals Work