I am creating an FFS after doing some filtering with a DGN file but along with the FFS file, my workbench is creating an FRS file as well. Why is this happening? And it doesn't happen with all the DGN files.

Hi @rishi1804​ ,

Rasters stored to an FFS file will have their data written to a corresponding .frs (FME Raster Store) file. One FRS file may hold the data for multiple raster features. 

FRS files hold up to 2 GB of raster data; if the file surpasses this size, the data is automatically split across multiple files. If an FFS file containing raster features is opened and the corresponding FRS file cannot be opened, then the raster features will be restored as polygons with attributes indicating the properties of the raster (for example, number of rows/columns, spacing, etc.).

As a result, you are likely seeing an frs file being created whenever a 3D feature with an appearance is detected or a drawing has an embedded image. If you want to prevent FRS files from being created you could use a GeometryFilter and filter for Raster geometry and send the <Unfiltered> port to the writer feature type.