typically x86-32bit or x86-64bit, this defines the processor type and support of your system. This is defined when your stage was built and should not be changed.
defines whether your system is a current, stable or experimental build. current systems will have newer packages unmasked than stable systems. This is defined when your stage is built and is typically not changed.
Defines optimizations for your CPU. The subarch is set at the time the stage3 is built, but can be changed later to better settings if necessary. Be sure to pick a setting that is compatible with your CPU.
defines the general type of system, such as server or desktop, and will set default USE flags appropriate for your needs.
define various optional settings that you may be interested in enabling.