You may get betters answers/descriptions in the “finishing” area of the forums but I have used steel wool in flat work. That is where I learned it. Almost all of the finishes that come in gloss, flat, etc have silica added to the product depending on the finish required; the higher the silica content the lower the gloss. So if you use flat you have a greater build up of the silica which diffracts the light. Some use a flat finish just for the top coat with undercoats clear. By using a high gloss and then cutting it back you can get the flat finish leaving the product free of silica. Use oil (to reduce friction) and a very light touch with the grain then wipe off. You can flatten poly the same way, make sure any finish is completely dry for flat work.
Use a high quality steel wood for uniform scratches. You are not changing the finish it is just as durable, you are just adding fine scratches to diffuse light. Frank Klausz made a very good video (1980’s) on finishing with one section on “finishing the finish” where he takes the same finish and takes it up to a piano type finish and down to a low luster.
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