yum Environment Groups?

[D. Hugh Redelmeier]

On my Fedora 18 system, when I do a "yum group list", I get three lists of 
groups: Available Environment Groups, Installed Groups, and Available 
Groups. I understand what the second and third lists are.

What is an Environment Group?  Why is none actually installed (eg.
GNOME Desktop)?

The yum manpage mentions Environment Groups obliquely, but doesn't explain 
what they are.  My googling doesn't seem to yield anything helpful.

After I wrote that, I continued playing and discovered some more
surprises.

I installed a bunch of groups (but no Environment Groups).  Now "yum
group list" shows an Installed Environment Groups list:
Infrastructure Server, Minimal Install.  These had previously been
listed as Available Environment Groups.

Next, I installed Environment Group "GNOME Desktop".  I'd have thought that 
was the default.  No matter.  A small number of additional packages
were added.  But yum list still shows "GNOME Desktop" as an Available
Environment Group, not an Installed Environment Group.  How come?

Similarly, I did a yum install of Environment Group KDE Plasma
Workspaces.  No packages actually got added since they were all
already there.  Yet yum group list still shows KDE Plasma Workspaces
as Available, not Installed.