@damorgue, ah, Phantom also support ISO layout but tmk_keyboard doesn't have code for ISO for now. You changed Phantom keymap file to support ISO keyboard. I get it now.
Your fix will be helpful for ISO Phantom people, send me patch or pull request when you have spare time.
OK. What I wanted to know is where BSLS(ANSI backslash) keycode is work for ISO key(next to Enter) or not. That key should have NUHS as HID spec indicates so. If you can edit keymap and build your own binary you should use NUHS. But If one binary with default keymap supports both ANSI and ISO keyboard it'll be helpful for someone who can't or don't want to build their own firmware. This is just the point of my last fix.
The key between Left Shift and Z has no alternative code other than NUBS. No question here.