NetBSD-9.99.91-amd64-bios-install.img.gz	14-Oct-2021 12:49	1251490kB
NetBSD-9.99.91-amd64-install.img.gz	14-Oct-2021 12:45	1251715kB

Is there any description on what is the difference above two ?

I've used the latter one to install to Let's Note CF-SX2 by writing to USB-disk,

zcat NetBSD-9.99.91-amd64-install.img.gz | sudo dd of=/dev/rsd0d bs=2m 

And its startup screen, it says 'bios boot'

By the way, the Note PC (built-in) LCD is black out. You need external LCD to start width. I usually make another modified kernel to use internal LCD.