Hi yanlin.li,
What’s your Jetpack version in use?
There are 2 kinds of redundant A/B system on Jetson Orin NX, one is Bootchain A/B which includes MB1/MB2/bpmp/bootloader..etc(flash_t234_qspi.xml) inside the QSPI and another is Rootfs A/B which includes kernel/rootfs/DTB..etc (flash_l4t_t234_nvme_rootfs_ab.xml) in external storage.
Bootchain A/B is enabled by default, and you can refer to Update and Redundancy for details.
Rootfs A/B is optional and you can enable it through adding ROOTFS_AB=1 in flash command, and you can refer to Root File System Redundancy for details.
Please also refer to the following threads for A/B redundancy use cases.
Query: Question about A and B partitions - #3 by KevinFFF
Logic: Reset Timing of Boot Retry Count - #7 by KevinFFF
Verification: Rootfs A/B redundancy fail-over mechanism in Jetpack5.1