The Cycles Tangent node defines a Tangent-Space for a mesh by either using one of the object’s local axes (see image A) or by using an available UV layout.
This can be useful when designing an Anisotropic reflection shader like in the case of brushed, machined or lathed metals.
A grey-scale texture can be connected to the Rotation input of the shader to define changes in the Anisotropic direction on the surface (see image B).
A. In this example the Tangent node is set to ‘Radial’ Direction mode, in which a object axis is chosen as the radial axis of the Anisotropic direction:
B. In this example a Noise texture’s Factor output is connected to the Anisotropic shader’s Rotation input to create an irregular Anisotropic (metal brushing finish) direction: