Instance specific context. This context is passed back to the web part in the onRender and onDispose APIs. The web part can use this context to manage state information.
An UNIQUE key indicates the identity of this contorl.
The PropertyPane uses ReactJS to render its components. ReactJS uses keys to identify a component and if it should be re-rendered or not. This is a performance feature in ReactJS. Please read the following link to understand how to pick the value of the key.
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Private properties of the PropertyFieldMapPicker custom field. We separate public & private properties to include onRender & onDispose method waited by the PropertyFieldCustom, witout asking to the developer to add it when he's using the PropertyFieldMapPicker.