You can choose which System folder to use by the
Startup Disk control panel.
Don't confuse the "System Folder" with the "System" folder, which is the one Mac OS X uses. You can switch to this using the startup disk control panel as well, or, on newer machines, holding down Option at startup. (doesn't always work for me)
The Contents of the Mac OS 9.2.2 System folder:
Appearance
Apple Menu Items
Application Support
Classic
Classic Support
Classic Support UI
Classic Update Log.txt
Clipboard
ColorSync Profiles
Contextual Menu Items
Control Panels
Control Strip Modules
Developer VISE Moved Items
Extensions
Extensions (Disabled)
Favorites
Finder
Fonts
Help
Internet Plug-Ins
Internet Search Sites
Language & Region Support
Launcher Items
Login
Mac OS ROM
MacTCP DNR
Panels
Preferences
PrintMonitor Documents
STF
Scrapbook File
Scripting Additions
Scripts
Servers
Shutdown Items
Startup Items
Startup Items (Disabled)
System
System Resources
Text Encodings
VolumeNameIconPict