To define an area that should not be vacuumed, you can use the "Virtual walls" function on the Xiaomi MI Robot Vacuum S12. This function allows you to exclude certain areas by creating virtual barriers in the app. Here are the steps:

1. app installation and connection: make sure the Mi Home app is installed and the robot vacuum is connected to your Wi-Fi.

2. map creation: Let the robot create a map of your home by having it drive through the different rooms.

3. create virtual walls: Open the Mi Home app and navigate to the settings of the robot vacuum. Look for the option "Virtual Walls" or "No-Go Zones" and create virtual barriers to mark the areas you want the robot to avoid.

4. save and test: Save the changes and test the function by having the robot drive through the areas in question.

This method allows you to precisely control which areas should be vacuumed and which should not.