When you first connect your Dyson machine to the Dyson app you'll give it a room name. You'll find a list of pre-set room names in the app, such as 'Bedroom' or 'Kitchen'. Google Assistant uses this room name to identify your machine.
Google Assistant uses this to name your machine in the Google Home app as well.
To control an individual Dyson machine using voice commands, you can either directly name the machine in your command or say the room name the machine is assigned to in the Google Home app.
If you say "Hey Google, enable auto mode in the Bedroom", Google Assistant will identify the machine named 'Bedroom' or a machine in the bedroom and enable auto mode.
Saying "Hey Google, enable auto mode", will enable auto mode for any purifiers in your current room, if the room is set up in the Google Home app.