Here is the procedure to boot an Asus x200 (x200ca in my case) from USB (ie. to install Linux on it).
While booting, press F2 to enter the bios
Go to the “Security” tab and disable “secure boot”
In the boot tab, enable CMS and Enable PXE boot
In the boot tab, disable fast boot
In USB options, set all to enabled.
Save your change the press Esc while rebooting. A boot menu should appear.
This worked for me on bios version 300-something. A Youtube video implied that you needed version 500 or above on some x200 version. This was not the case for me.
I was able to boot into a Manjaro Linux live distro on USB and everything worked great. In particular the touch screen seemed to work better than under Windows (I seemed to get a lot of false touches under windows).