Clarification on how Elmah set the Logged in User Name

Clarification on how Elmah set the Logged in User Name

I was searching on how the logged in user name was set using ELmah.
The closer i came is the below link
Logging username with Elmah for WCF Webservices
Elmah take the user from Thread.CurrentPrincipal.Identity.Name and not
from HttpContext.Current.User
To get more clarity on this does the Thread takes Name from the windows
log on ?
Or will it log the name if i set HttpContext.Current.User ?
How does it work. How application architecture should be to make ELMAH
feasible to log UserName ?
Thanks