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Add a parameter to the User tab under either the Attributes or Primvars section in the RenderMan tab. The parameter must be located within the User folder and must be prefixed "ri_user_" to be read. 

You must also supply a Tag Value to the parameter to inform Houdini and RenderMan of the type and allow interactive edits.

Setting up MatteID as an Example

Create a color attribute called "ri_user_MatteID0". 

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