What are DND mode, Smart DND mode and Timed DND?

Turn on DND: once turned on, the watch will no longer vibrate when notifications and incoming call reminders are received from your mobile phone. Tap again to disable DND mode.
Timed DND: You can customize the time period during which DND mode is automatically turned on or off.
Smart DND: After you fall asleep with the watch, the watch will automatically turn on DND mode.