There are two separate fuses defining the BOD configuration in different sleep modes; One fuse defines the mode used in Active mode and Idle Sleep mode (ACTIVE in FUSE.BODCFG), and is written to the ACTIVE bits in the Control A register (BOD.CTRLA). The second fuse (SLEEP in FUSE.BODCFG) selects the mode used in Standby Sleep mode and Power-Down Sleep mode, and is loaded into the SLEEP bits in the Control A register (BOD.CTRLA).
The operating mode in Active mode and Idle Sleep mode (i.e., ACTIVE in BOD.CTRLA) cannot be altered by software. The operating mode in Standby Sleep mode and Power-Down Sleep mode can be altered by writing to the SLEEP bits in the Control A register (BOD.CTRLA).
When the device is going into Standby Sleep mode or Power-Down Sleep mode, the BOD will change operation mode as defined by SLEEP in BOD.CTRLA. When the device is waking up from Standby or Power-Down Sleep mode, the BOD will operate in the mode defined by the ACTIVE bit field in BOD.CTRLA.