I googled a while but all I could see was USB /SD card install for RPi, and the install doc has no mention for how to create a USB boot for a native x86-64 PC without having to install DietPi on SSD/HDD:
So how to do USB boot for PC? Or do I need to install on an internal /external SSD/HDD ?
Just put a USB drive you want to install the OS to…then boot another USB bootable drive with DietPi installer…install to the “static USB” the OS will be on…shutdown, remove the USB bootable drive, set the BIOS to boot from the “static USB”
Probably we have misunderstood each other. If you have a bios system, download the Direct Write Image and flash it directly onto a USB stick with Rufus. After that you should be able to boot from it.