I am modelling a friction stir welding process, and the surface film condition is applied on the exterior surface of the eulerian domain. I think the surface film condition should be applied to the real materials surface in the eulerian domain, not the exterior surface of the eulerian domian. How to define the real materials surface in the eulerian domian with Abaqus/CAE?

I found the following eulerian surface definition.

"Data line to define surfaces created with TYPE=EULERIAN MATERIAL
First line
Name of the material instance as defined in the EULERIAN SECTION.
Abaqus/Explicit will automatically create a surface on the exterior of the given material."

How to use this eulerian surface to define surface film condition?