The layer bar (and the object lister and the layer stats) shows the layer information. The layers can be arranged visually by click drag MMB either to left and right. Additionally the layers can be grouped together to form categories. Default categories are All layers, Reference and Selected Layers. Additional Categories can be made by clicking on the categories and choosing editor.
Categories are very useful in cases where large number of layers are present. By selecting the categories, only the layers in that category are visible. It is analogous to layer groups in other CAD packages.
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