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MaterialX is a ASWF standard for the material definition and exchange between renderers.  It provides definitions around a set of Physically Based Shading Nodes, as well as a set of pattern nodes (like texture nodes, procedural noise, etc) that you can use to build up a material.  Each renderer that supports MaterialX takes the material definitions and either renders them directly or converts them to an internal representation.  Because there are many renderers in this world, each approaching the challenge of rendering from a different perspective, MaterialX doesn't guarantee a pixel match between renderers. 

MaterialX only provides guidance to each renderer, and each renderer will do their best to match the intent of the material in the best way that they can.

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The Physically Based Shading Nodes you use in your MaterialX graphs are all converted to PxrSurface and then passed to RenderMan.  PxrSurface can represent most of what MaterialX defines.  Because each renderer is different, the picture you get from RenderMan with a given MaterialX network may vary from the picture you get from another renderer with the same network.  One limitation that currently exists is that we don't support material layering via MaterialX.

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