It does have a front thread, allowing another filter to be mounted in front of the Black Soft filter, as long as the aperture is the same. 

Compared to uncoated filters, coating effectively reduces reflections, thereby minimizing flare and ghosting. 

Different apertures have different outer diameters. You can stack filters by simply purchasing filters of the same aperture (e.g., a 58mm Black Soft filter can be layered with a 58mm ND filter). 

1 has the strongest soft focus effect and the most pronounced haziness. It is recommended for scenes with high contrast between light and shadow or a narrow highlight range. 1/2 has a stronger soft focus effect and a more pronounced haziness. It is recommended for scenes with high contrast between light and shadow or scenes with high contrast between light and shadow. 1/4 has a relatively mild soft focus effect and a moderate soft focus range, making it more versatile and suitable for scenes with low contrast between light and shadow. 1/8 has the mildest soft focus effect, and sometimes it may not be noticeable when the highlight range is narrow. However, this gentle soft focus also gives it the greatest versatility, allowing it to be used effectively in a wide range of light conditions. 

Different levels of the Black Soft Filter gradually soften highlights in the image, softening the transition between light and dark. The higher the value, the stronger the softening effect.