Looking at the Node Group in detail you can see that the Specular socket of the Group Input node is connected to the Color1 socket of the Mix node. Clearly the Specular socket is a color field.
Question I don't understand how it is possible that while the Specular socket is a color socket it is possible to enter values in the Specular socket. If I create this node group this is my Reflection Node Group. In my Specular socket you can only enter a color and not a number.How can I change the Specular socket from a color socket to a "value" socket ?