In class, we used the OSCDIMG command to create a Windows PE image a question came about as to when the –b switch does. According to Microsoft, the –b option specifies the location of the El Torito boot sector file.
From Wikipedia, the El Torito Bootable CD Specification is an extension to the ISO 9660 CD-ROM specification. It is designed to allow a computer to boot from a CD-ROM. In short, this helps us create a bootable image.