https://www.macrumors.com/2009/08/12/os-x-snow-leopard-installation-process-changes-described/
- Options to "Erase and Install" and "Archive and Install" are no longer present in the Mac OS X 10.6 installer. According to those familiar with the software, this was done for convenience, so that users do not accidentally erase and install their Mac OS X 10.5 Leopard partitions. However, "Erase and Install" remains available through Disk Utility, which is also included on the installation DVD.
- If you need to reinstall 10.6, it automatically archives and installs for you.
- Options to "Erase and Install" and "Archive and Install" are no longer present in the Mac OS X 10.6 installer. According to those familiar with the software, this was done for convenience, so that users do not accidentally erase and install their Mac OS X 10.5 Leopard partitions. However, "Erase and Install" remains available through Disk Utility, which is also included on the installation DVD.
- If you need to reinstall 10.6, it automatically archives and installs for you.