The User Scalar node texture allows the user to read and return a single number value of the currently shaded object. This can be useful if the same material or texture is applied to several objects in the scene, but we still want to control some of its aspects (e.g., material glossiness, a blend between two other textures inside a Mix texture, etc.) with user-defined properties. Visit the User Attributes Workflow page for information and tutorials on user-defined properties. 

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User Attribute – Specifies the name of the user attribute that is looked up and returned from this texture.

Default Value – Specifies the value to return if the user attribute is not found on the shaded object.