Hi abdoubouam,
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Each object is assigned a single slot containing multiple materials.
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For each object that is assigned a slot, 1 of those materials in the list will be assigned to it
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When Grungit is applied, it applies itself to the entire list of materials in a slot, instead of the one material within the list that is assigned to the object.
Here are examples of the setup (I chose objects with the simplest materials as to not clutter my explanation):
