Serna GUI Layout is the current layout of the GUI (buttons, menus, short-cuts, location of buttons and dockable dialogs). If you think the GUI needs some customization like changing the location and appearance of the GUI controls or changing the hot keys, you can easily do it.
For example, you can specify your own icons for the Redlining buttons and move the Redlining toolbar to another location. If you do not need the Redlining toolbar at all, you can remove it. Note that the changed layout will be active only for the current document. If you want your changes to be persistent, you must save them (see Saving Customized Serna View).
For instructions on how to customize the GUI layout, see How to Customize GUI Layout, How to Override Icon and How to Change Hot Key.