Say you wanted to build a new pentium4 stage3 tarball. The recommended method of doing this would be to grab an existing pentium4 stage3 tarball to use as your seed stage. Metro will be told to use this existing pentium4 stage3 to build a new stage1 for the same pentium4. For this process, the generic pentium4 stage3 would provide the build environment for creating our new stage1. Then, the new stage1 would serve as the build environment for creating the new pentium4 stage2. And the new pentium4 stage2 would serve as the build environment for creating the new pentium4 stage3.