Symbols can be grouped by layers. Each layer can be shown or hidden individually. By drawing a new symbol, it is added to the active layer.
When selecting multiple symbols at a time, only the symbols at the active layer are selected. When you click on a symbol which is not on the active layer, you automatically switch to that layer.
The symbols which are not on the active layer are faded away, so you easily see on what layer you're working in. In the "View" menu you can click "Brightness" to change how much the symbols from the non-active layers are faded.
Select one or more symbols, and click with your right mouse button on one of the selected symbols. Choose "To other layer..." in the menu which pops up to move the symbols to another layer.
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