The core principles of the standard make behaviour more defined and debuggable and gives BIOS manufacturers room to further dynamise boot device selection for the user on top of suggestions of the standard.
It then rescans the region after all the PnP option ROMs have been initialised because, as appendix E states, the option ROM initialisation routine may have chained more PnP expansion headers for individual disks the device owns. The BCV, however, hooks interrupt routines which interact with the device which are adjusted based on a base MMIO address location, disk information ascertained in the option ROM initialisation routine and the current disk number in the BDA.
Before it had hooked the interrupt there may have been no disks on the system, but by intercepting the interrupt and altering the values returned, the SCSI Option in rom option in rom make all the disks on the SCSI bus visible option in rom the system.
Multiple controllers can hook INT 13h at once. The option ROM contains the program required to download the boot code. INT 19H is called to initiate the boot process, while INT 18H is called when the system tries to boot from all possible devices and none were bootable.
By hooking INT 19H, the system would attempt to boot from the network before any other devices. Note that when Secure Boot is enabled, execution of the Compatibility Support Module and legacy Option ROMs is prohibited because legacy firmware drivers do not support authentication, which is a security threat.