BSV1 and BSV2 are the boot security vectors which are evaluated during execution of Boot Rom after a reset.
They contain alternative boot vectors (reset vector) to be executed after Boot Rom is completed.
The standard boot vector for MB(CY)91F460 series is Flash address 0xF4000. It is executed in case there is no valid value written in BSV1.
Valid value means a Flash address between 0x00040000 and 0x0014FFFF. In case the sector where BSV1 is located (Flash address 0x00148004) is erased, BSV1 value is 0xFFFFFFFF which means it is invalid.
In that case the default boot vector at Flash address 0xF4000 is executed and your bootloader might not be started, and re-programming is required via UART4 Flash programming interface.
For more details please refer to chapter 53.2.2 in MB91460 Series HARDWARE MANUAL.