When the DAC is enabled (ENABLE=1 in DACn.CTRLA), a conversion starts as soon as the Data register (DACn.DATA) is written.
When the DAC is disabled (ENABLE=0 in DACn.CTRLA), writing DACn.DATA does not trigger a conversion. Instead, the conversion starts on writing a '1' to ENABLE in DACn.CTRLA.