When you start BCA assistant, one Macs that support a USB Installer,
1. BCA erases and formats the USB to FAT32.
2. Downloads the BC Drivers and creates a WinPE (Preboot folder with drivers needed to support the Windows installer). This environment is specific to the hardware, not the specific version of Windows. BCA 5.1 will support W7 32-bit and 64-bit, W8 and W8.1 64-bit.
3. BCA then prompts you for the location of the Windows Installer media ISO.
4. The contents of the ISO, after signature verification by BCA, determines the version of Windows that the user chooses to install. This is a user selection, not a random version determined by BCA.
5. BCA sets the NVRAM and reboots and hands control over to the Windows Installer.
On Macs which support only DVD,
3. Uses the inserted DVD to determine which version, based the catalog files, it will install.
4. BCA boots from the physical media in the Optical drive as chosen by the user after setting appropriate NVRAM variables.