Here you must specify the file containing data from a hydrodynamic simulation made with MIKE 21 HD. You can type in the file path or click the file chooser on the right and navigate to the file.
Once selected, MIKE 21 ST will validate that the file is actually a data file from MIKE 21 HD.
Please note that the size of the grid cells used in the hydrodynamic simulation is mirrored directly in the simulation made by MIKE 21 ST, so make sure to account for this when setting the parameters for the sand transport simulation.