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The Shader Mixer Pane (Tab) is included in the Advanced and Pro editions of DAZ Studio v4.x. Stated simply, it allows you to create your own custom shader by connecting a series of bricks together, to form a shader_network.
Like Shader Builder (WIP), Shader Mixer provides a visual development environment for generating shaders based on the RenderMan® Interface Specification (RISpec), but it does it at a much higher level. Although functionality to integrate a block you create in Shader Builder exists, with Shader Mixer you don't actually have to write code or care about syntax and things like type_conversion are handled for you, in the background.