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The dummy module is an instance, that does not provide its own function or status variables, but can be used as a placeholder only to get a representation as a device in the visualization.
So it is conceivable that you create several variables that belong to a device for which IP-Symcon provides no module. To represent these variables within an unit together in the visualization (e.g. WebFront) you can create the dummy instance and place the dummy variable in the instance.
In combination with the variable profiles and the variable actions you can create your own "instances", which have similar functionality in the function and automatic visualization of virgin instances.