BC_Type = FluidFluidInterface: This boundary type is used to describe interior interfaces among different fluid materials. These occur when solving co-extrusion problems. If free surface calculations are turned on, the deflection of this interface will be calculated. Additionally, if mesh update is turned on, this interface along with the mesh will be updated based on the calculated displacements.
HyperXtrude can handle more than one material being extruded simultaneously. Interfaces among these materials are designated with this specific BC.
The syntax for this boundary condition is:
FluidFluidInterface FFinterface { }
Here, FFinterface is the name of the fluid interface boundary and is specified by you.