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You may assign a PxrMeshLight material to an object to create a mesh light. However, you will not be able to take advantage of light linking and light filter options.
Additional Notes
The geometry can be rendered with or without a Bxdf. RfK assigns PxrBlack to mesh lights if no other Bxdf is assigned (does not apply to Method 3). If a Bxdf is assigned to the geometry then the geometry will be rendered according to the characteristics of that shader.
It is recommended that you increase the micropolygonlength on the mesh light for improved performance. This attribute is available on the PxrMeshLight GafferThree package in the Object Tab.
If you want to use the Texture Color parameter of PxrMeshLight, you can create a shading network with PrmanShadingNodes and a NetworkMaterial node. Then assign the NetworkMaterial to the light location with a MaterialAssign node, downstream of the GafferThree where the light was created.