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In food, cellulose is the scientific term for dietary fibre. It can be found in the cell walls of all plants and is almost entirely indigestible for dogs which is actually a very good thing as its undigested form provides a wide range of benefits.
However, while cellulose per-se is good for dogs, it's usually not a great thing to see it listed as an ingredient. This is because pure cellulose or lignocellulose powder is quite different from the fibre found in whole fruit and veg. It is a white, odourless powder that is most often produced as a low value byproduct of agriculture (e.g. corn stover, sugarcane bagasse, straw) or as waste material from saw mills and paper mills (it really is!).
Cellulose is most often found in light diets since it helps to bulk out the diet whilst providing almost zero calories.
As fibre supplements go, cellulose powder is probably the most controversial and many nutritionists recommend avoiding it altogether.