Mac can boot OSX from an external, but I dunno whether a Mac can boot Windows from an external. In general, Windows can't be boot from an external.
A Bootable USB Flash, is exactly that, the flash has been formatted with a bootstrap loader and contains an OS to boot with, NOT Windows. Don't ask us, ask Microsoft.
Rufus, one of its popular function is to boot with a Windows install ISO, so you can INSTALL Windows from a flash, not requiring a CD/DVD.