Stage 1 tarballs are generated from a packages.build file. Each system profile may have a slightly different packages.build file:
Stage 2 tarballs contain the same packages as a stage 1 tarball with one caveat: stage 2 tarballs are compiled from a stage 1 tarball. This is to ensure the stage 1 tarball contains the tool chain necessary to reproduce itself.
Stage 3 tarballs are compiled from stage 2 tarballs, but contain a system set. Portage includes a quick reference to this set of base packages by using the @system set operator.
Not including sub-profiles, the base file used for the system set of all profiles can be found at: /var/db/repos/gentoo/profiles/base/packages
Downloading and decompressing a stage 3 tarball as described in Gentoo Handbook will install an almost-complete and almost-functional system (the most important parts still missing are a kernel and a bootloader).
Official stage 4 tarballs became available in January 2016 for the amd64 architecture on distfiles.gentoo.org. These tarballs contain a kernel and bootloader and therefore provide a bootable system. No automatic network detection is attempted, however, so they are not intended to replace the usual Handbook-based installation method starting from a minimal installation CD or Gentoo LiveDVD and a stage3 tarball. (The LiveDVD can be used to get a full, working Gentoo desktop system without first installing Gentoo. Users with access to the appropriate commands can also create a LiveUSB.)
A cloud stage 4 has been created to aid in the process of virtual machine provisioning. These stage 4 files can be used with diskimage-builder (available via app-emulation/diskimage-builder). See the Gentoo README upstream and official diskimage-builder documentation for more information.
- http://www.funtoo.org/Stage_Tarball - Stage tarball explained on the Funtoo Linux wiki.