In addition most Macs will not run an older operating system than they shipped with as this article states:
So this means the last retail 9.2.1 disk (a white disk with orange 9 logo that does not say Update) will only install on Macs which shipped with 9.2 or earlier which are G3 or better, and the 9.1 retail disk will only install on PowerPC Macs which shipped with 9.0.4 or earlier. Pre-G3 Macs won't run Mac OS X without special third party software. For best Classic compatibility, upgrading to 9.2.2 using this article helps if your Mac is among the older ones which can use those retail disks:
In 2003, new Macs started being released that wouldn't boot into Mac OS 9 as outlined by this article:
While no longer being able to boot into Mac OS 9, they could still use the Mac OS 9 system folder for the Classic environment within Mac OS X. Starting in 2006, the new Intel Macs no longer included the ability to restore Mac OS 9 since Classic does not run on Intel Macs. Here is a listing of articles which describe the variety of restore applications by restore disks from Macs released between 2002 and 2005:
Mac OS X 10.4: Restoring applications from a Mac OS X 10.2 Software Restore disc - Apple Support
Using Restore Discs with Mac OS X 10.2 through 10.3.3 - Apple Support
If anyone here knows which specific models and vintage of Mac model followed which of the specific instructions above, that would be helpful as addition to making this User Tip, so users know which article to refer to, if they ever need to erase and install Mac OS X and restore the Classic environment and/or Mac OS 9 booting to their Mac if it is supported.