ACF files are so-called Adobe Photoshop Custom Filter files, so filter files for the image editing software Adobe Photoshop. When calling the Custom Filter via 'Filter-> Other Filter-> own filter', a matrix appears in which one can enter values for so-called weightings for scaling and offset.
Using this matrix and the underlying mathematical functions, you can achieve many different effects, such as emboss, invert, or sharpen. If you have created a usable value matrix, you can also save it. The created ACF file can later be reloaded or used for batch processing.