The Address Match unit is used only in slave mode, and checks if the received address bytes match the 7-bit address in the TWI Address Register (TWAR). Upon an address match, the Control Unit is informed, allowing the correct action to be taken. The TWI may or may not acknowledge its address, depending on settings in the TWCR.
Although the clock system to the TWI is turned off in all sleep modes, the interface can still acknowledge its own Slave address or the general call address by using the TWI Bus clock as a clock source. The part will then wake up from sleep and the TWI will hold the SCL clock low during the wake-up and until the TWINT Flag is cleared.