Teach Me Layers

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So I understand you have a trans tile, layer 0 is the definitions for swimming, walking, or blocking, you can overlay with layer 1 on swimming or walking to look how you want it to and keep the swim/walk definitions, and blocking tiles keep definitions on all layers, so why would you need a layer 3, 4, or 5? I don't understand anything past that and if someone wouldn't mind explaining please.....