Apps can extend existing tables with extra columns to store additional data.

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Usage

In your app manifest.json, you can define extra columns for a table by adding a extra_columns property like this: 
...
  "extra_columns": [
    {
        "kind": "user",
        "columns": [
            {
                "name": "appx_myapp_column1",
                "type": "varchar",
                "size": 128,
                "is_required": false,
                "description": "Example of extra column in the 'user' table"
            }
        ]
    }
  ]
...
The extra columns should follow the Table Column schema.
Extra columns are removed from the database when the apps are deleted.