installing by group versus installing by individual packages

   trying to clarify a couple things regarding installing/removing
by yum group on my fedora 19 system. first, what is the rationale
behind "hidden" groups? what's the motivation that some groups
(for the case i'm interested in, virtualization) not be user visible?

   second, if i haven't installed the virtualization group, i can
query it and i see:

Group: Virtualization
  Group-Id: virtualization
  Description: These packages provide a virtualization environment.
  Mandatory Packages:
    +virt-install
  Default Packages:
    +libvirt
     libvirt-daemon-kvm
     qemu-kvm
    +virt-manager
    +virt-viewer
  Optional Packages:
    +guestfs-browser
    +libguestfs-tools
    +python-libguestfs
    +virt-top

that seems fairly straightforward -- i'm seeing what would happen
if i installed that group. '+' means packages that would be
installed, while no symbol represents packages that are already
installed. and after i install by group, i get:

Group: Virtualization
  Group-Id: virtualization
  Description: These packages provide a virtualization environment.
  Mandatory Packages:
    =virt-install
  Default Packages:
    =libvirt
     libvirt-daemon-kvm
     qemu-kvm
    =virt-manager
    =virt-viewer
  Optional Packages:
    -guestfs-browser
    -libguestfs-tools
    -python-libguestfs
    -virt-top

where i assume '=' means that package was installed by that
group install command, while '-' means not installed (optional).

   however, if i had installed all those virtualization packages,
not with a single group install command but by explicitly
installing each package, i would have ended up with:

Group: Virtualization
  Group-Id: virtualization
  Description: These packages provide a virtualization environment.
  Mandatory Packages:
     virt-install
  Default Packages:
     libvirt
     libvirt-daemon-kvm
     qemu-kvm
     virt-manager
     virt-viewer
  Optional Packages:
    +guestfs-browser
    +libguestfs-tools
    +python-libguestfs
    +virt-top

which would have had the same effect but, i'm guessing,
from the perspective of yum, the virtualization group is
*not* installed. so, for instance, i can't remove those
packages using a "yum groupremove"; i would have to do it
explicitly again. do i understand this vaguely correctly?

   if there's a good explanation of this online, a URL would
be dandy, thanks.

rday