gentoo: What is Global Update?

tmp gentoo 
# emerge -vuDN world

  (snip)

Total bytes received: 6306455

sent 82347 bytes  received 6306455 bytes  43022.24 bytes/sec
total size is 181408409  speedup is 28.39


Performing Global Updates: /usr/portage/profiles/updates/3Q-2009
(Could take a couple of minutes if you have a lot of binary packages.)
  .='update pass'  *='binary update'  #='/var/db update'  @='/var/db move'
  s='/var/db SLOT move'  %='binary move'  S='binary SLOT move'
  p='update /etc/portage/package.*'
....................................................

google:gentoo /etc/make.profile
Upgrading Gentoo - Gentoo Wiki
Gentoo X86 Handbook - Gentoo Wiki
google:Performing Global Updates gentoo
google:gentoo Updating to 3Q-2009
http://wiki.gentoo.gr.jp/index.php?Portage%E8%A8%AD%E5%AE%9A%E3%83%95%E3%82%A1%E3%82%A4%E3%83%AB#na30f185