The K&F Concept 3-in-1 is a filter that combines variable ND (ND2-32), circular polarizer (CPL) and Black Mist 1/4
For those who are not practical: ND stands for neutral density, by rotating the ring you can lower the brightness that enters the lens, simply unscrew the tightening screw slightly and rotate the ring, on which the numbers are written.
CPL: stands for “Circular Polarizing Filter”, a polarized filter, so it removes reflection and reverberation, this should lead to slightly more saturated colors, and for example, by photographing a body of water, you should be able to view a part of that is under water, obviously based on clarity.
Black Mist: on the other hand, it is a filter used for high lights, the classic example of a flame in the dark that results in a reflection diffused all around.

The product looks good, with plastic packaging and there are 3 cleaning cloths.
The filter is made of excellent aluminum, the rotation system for adjusting the ND is fluid and of the right hardness.
I tried the 82mm on a Sony 24-70 GM2 lens, and I noticed a bit of vignetting, unfortunately this is one of the most well-known problems with this type of filter.
The polarization is good and since I already own a 1/4 Black Mist filter from this brand, I can say that the effect is the same.

In conclusion, it is a convenient filter because it contains several filters together, and rather than having 3 overlapping them, 1 is better.
If you need to create this slightly cinematic effect, it is a product to evaluate, it can do that effect on highlights and give more saturated colors, and ND allows you to manage the intensity of the light.
Watch out for vignetting, especially with the filter completely closed.
Pass the tests with 5 stars!