Looks like processor architectures to me. SGI (Silicon Graphics inc.) used 32 and 64 bit MIPS processor I think, i86 and equivalents means Intel 32 and 64 bit x86 processors. AMD64 is Advanced Micro Devices
There are also Sparc (Sun/Oracle) and PowerPC (IBM) processor architectures out there along with others; Itanium, Transdata, ARM, etc.
So you'd have an SGI platform, an x86 platform, a Sparc platform, etc, etc.
ICL Superservers and Teamservers were Sparc platforms.
If you want to install Gentoo, you have to download the distribution for the correct platform.
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