This is how I could see tags working: Add a "Layer Tags" panel.  Layer tags are named, in a flat namespace (no hierarchy needed), and global to the entire document (independent of spread or artboard). Any given layer can have one or more tags associated with it.  A layer which has one or more tags associated with it would show a tag icon in the Layers panel, and the tag icon would be a different color when that tag is selected in the Layer Tags panel, to help identify which layers are