The layers shown in the layer list are rather "pointers" to the actual layout layers and not representing the actual layers. Because of this, these layers are more precisely referred to as "views". Layers can be removed and created again without affecting the actual layout data.
To create a layer, choose "Insert View" from the layer context menu (right mouse button click on the layer list). Then, an input dialog prompts for the source specification. The source specification tells, from which actual data layer to take the displayed data from. The most simple form of a source specification is "layer/datatype" (i.e. "5/0") or the layer name, if an OASIS layer name is present. This specification can be enhanced by a layout index. The first layout loaded in the panel is referred to which "@1" or by omitting this specification. The source specification "10/5@2" therefore refers to layer 10, datatype 5 of the second layout loaded in the panel.
Source specifications can be wildcarded. That means, either layer, datatype or layout index can be specified by "*". In this case, such a layer must be contained in a group and the group parent must provide the missing specifications. For example, if a layer is specified "10/*" and the parent is specified "*/5", the effective layer looked for will be "10/5". Unlike the behaviour for the display styles, the children override (or specialize) the parent's definition in the case of the source specification.
The layer list can be cleaned up to remove layer views that do not correspond to actual layout layers using the function "Clean up views" from the context menu. Similar, layers that are present in the layout but for which there is no view can be added using the "Add other views" method.