The A/B partition setting indicates that there are two systems A and B in the ECU, and both systems are mutually redundant.
During normal operation of the ECU, system A is operating normally and system B is in standby state. When the ECU receives an upgrade campaign, system B starts to upgrade, while system A is still running normally.
After system B is upgraded and the ECU reboots, system B will start running normally, and system A will enter the standby state instead.
If system B fails during the upgrade process, the operation of system A will not be affected, and the ECU can still work normally.
This design improves the safety of ECU upgrades and avoids situations when the hardware cannot operate normally due to failure of the update.