Normally in the Camera view, any visible line or drawing can be selected. When you enable the Works on Single Drawing mode, only drawings from the currently selected layer can be selected.
The Apply to Synced Drawing Layers option works in conjunction with the Works on Single Drawing mode. The Works on Single Drawing mode still limits the selection to a single layer, but the Apply to Synced Drawing Layers option allows for the inclusion of drawings on layers synced to the selected drawing layer.
Layers are synced when drawings need to be separated on different layers, but need to have the same timing.