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.
The palette commands listed here are 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 the following, the term "your Presets directory" is often used. You can access this by going to "Edit" in the main menu and selecting " Preferences". In the window that appears, you will find an Open Preferences Folder button at the bottom left. This will take you to this folder in Explorer or Finder.
In order to be able to access different layouts and menus as quickly as possible, all the layouts supplied and those you have saved yourself (see also Save Layout as...) layouts are displayed.
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 preferences 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 are also listed in this menu if they are saved in the prefs directory of your preferences folder.
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.
Incidentally, the set option is saved permanently in the CINEMA 4D.prf file in your preferences directory.
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 your preferences directory. Render settings 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 (only in the 3-point menu top right)
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.