TFS and build.vcs.number

I am just getting started with TeamCity.  We are connecting it to TFS for the VCS.  I am trying to figure out what the %build.vcs.number% value refers to in TFS.  It is supposedly the revision number of the VCS ROOT, but I cannot found the corresponding value in TFS. What I would like to do is to get the last changeset number applied to that root.  That number would be more useful for me.

Thanks,

-tj