I'm new to FASM.

This topic refers to using fasmg:
https://board.flatassembler.net/topic.php?t=20939

In order to use fasmg for Microsoft Windows x64, is fasmg or fasm2 the correct tool? (I do understand that fasmg is not CPU-specific.)

Thank You

Both are the same tool, difference is the headers included. But fasm2 has a better maintained set of headers specific for x86-64. The fasmg also has x86-64, but it is bundled as an example for x86 architecture. If you are in doubt, do as I did: install both and use included headers to study and learn the fasmg language.

Thank you. I'm gathering that one who seeks full-fledged OOB capabilities but also seeks knowledge and customization ability etc. would use fasmg, and use fasm2 as a reference from which to create their own fasmg config based on their focus. E.g. If not migrating from fasm1, one might not need certain aspects of fasm2.

I'm also interested in future potential support for ARM64 and fasmg appears intended for such expansion.

Interesting aside:
https://learn.microsoft.com/en-us/windows/arm/apps-on-arm-x86-emulation#how-emulation-works

Like Jessé said, fasm2 is just fasmg with a different set of headers. If you choose not to include these headers in your code, your experience will be exactly the same as if you were using plain vanilla fasmg.