Files with NB0 extension are primarily the ROM foot print.
We would be referring to primary bootloader for a AM335x processor. The first 4-Bytes of the image contains a jump to location address which is nothing but equals to 4K offset from the starting location.
The nb0 image also contains a ROM Signature "CECE", ROM TOC Pointer offset which is at an offset of 0x44 and ROM TOC Offset which is at a offset of 0x48.
The advantage of nb0 file is it can be downloaded to SRAM present on device and the Program Counter can be set to the starting address of the image. After wards either we can do step debugging to see which instruction is causing issues to boot the device from X-Loader and then we can trace back to the C source code.