Meaning "pc" in the triplet structure of the target architecture
You can find some advice on the GNU coding standards where you can read:
The configuration script must also take an argument that specifies the type of system to create the program. This argument should look like this:
For example, an Athlon based GNU / Linux system might be "I686-pc-gnu-gnu. The configuration script must be able to decode all plausible alternatives to describe the machine. So 'Athlon-pc-gnu / linux will be a valid alias
Also, i686 conventionally refers to 32-bit kernel, while x86_64 refers to 64-bit kernel, as you can see in this SO.
Especially instructive is the config.guess shell script , which shows all the features that count when the build type is not specified by an option or argument.
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