Reply To: Difference between System Coupling Region & Fluid Solid Interface

[Rahul Mule]

Both are used to identify the regions where the transfer of loads to and from external fluid solvers CFX or Fluent occurs.

Refer user guide to know more about it.
Fluid Solid Interface (ansys.com)
System Coupling Region (ansys.com)

You can use either of them, but using System Coupling Region is recommended. 

You should select the faces of the geometry that form the interface between the structural model and the fluid model. Refer tutorial to get more clarity on this: Create the System Coupling Region (ansys.com)