For example here is how it occurs in .installed.cfg for ERP5 SR:

    [jupyter]
    eggs = jupyter
            jupyter_client
            jupyter_console
            ...
            jupyterlab
            jupyterlab-launcher%(__buildout_space_n__)s

and without the fix it breaks as

    configparser.InterpolationMissingOptionError: Bad value substitution: option 'eggs' in section 'zzz' contains an interpolation key '__buildout_space_n__' which is not a valid option name. Raw value: '\naaa\nbbb%(__buildout_space_n__)s'