For the fluid structure interaction, a loosely coupling method is used. For the fluid part, the CE/SE method is used based on Eulerian frame and the structure is treated as an embedded moving boundary, while in the structure, a FEM solver is employed based on Lagrangian frame. Both fluid and structure meshes are independent (more accurate if the fluid mesh size is smaller than the structural mesh size).
The interface boundary locations and velocities are dictated by the Lagrangian structure. That information will be used by the fluid solver as the interface conditions at each time step, and it will feed back the fluid pressures (forces) on the structural interface as exterior forces for the structure solver.
In the future, when the automatic volume mesher option was implemented, all FSI boundaries will be Lagrangian and deform with the structure allowing exact imposition of boundary conditions.