Arnold Operators can be imported and exported by right-clicking on the Arnold output.
Tutorials about Operators can be found here.
Example use of Operators in HtoA
In HtoA, Operators are exposed as ROP nodes. Connect an Operator chain as an input of an Arnold ROP to apply the Operator chain when rendering from that ROP.
An Import/Export Arnold operators menu is available in the Arnold ROP. This enables you to export the connected ROP network as a .ass file (left) and import it back onto a different Arnold operator (right).
The Assign Material ROP creates an HDA that wraps the Set Parameter ROP for material assignment. This can be used to change the surface, displacement or volume shader on a selection or to override shaders in an alembic procedural for example. The Assign Material ROP does the following: