Customize
Under this menu item you will find a number of functions that allow you to manage various configurations of your working environment. You can find more information here: The user interface.
Some of the commands listed here are already described under The command palettes.
You can call up various main menus in the lower area. You can freely design the User X menus yourself (see The Menu Manager).
The relationship between the selected layout and the main menu is saved when you exit Cinema 4D. You then always have the right main menu for the respective layout.
Layout commands
The following commands are also displayed under the 3-dot icon at the top right: 
In order to be able to access different layouts and menus as quickly as possible, all .l4d files located in the library/layout directory within the program folder are displayed here.
Once you have set up your working environment as desired, you can save this state (but only that of Cinema 4D - you still have to tidy up your desk manually).
Use this command to load a previously saved layout (.l4d).
You can use this command to save the currently visible working environment as the default setting. This will then be automatically restored the next time you start the program (the same applies to your writing style as described under Load layout ).
The Start layout is saved as template.l4d in the Prefs folder of your user directory.
You can use this command to save the currently visible working environment to disk under a separate name so that you can switch between different layouts later, e.g., one layout for modelling and one for animation.
Layout files are given the extension .l4d and, if they are saved in the prefs directory in the Cinema 4D folder, are also listed in this menu.
If this option is activated, you can no longer change the current layout. This option is a protective measure against unintentional changes to the layout.
The set option is saved permanently in the Cinema 4D.prf file (in your prefs folder).
If you want to set the current scene as the scene that automatically appears in a newly created document, this command can be called up. It then saves a new.c4d file in the default settings directory. Render presets are also stored in this so that, for example, a specific frame-rate can be permanently set. Or a specific light setup can be defined in this way.
Old layouts
In Cinema 4D R25, both icons and layout were changed. So if you are switching from such an older version to a modern Cinema 4D version, this option will help you to get used to the new icons. This is because the layouts familiar from Cinema 4D R23 are offered at the top right above the main menu. This allows you to place the new icons in the old positions.
By deactivating Old layouts, the current layouts are displayed again.