Difference between config_extend and config

I setup a fresh setup today. I have some problem with spam in gmail so I want to change the credit in email html from image to text.
I change in config_extended.php for the setting to enable this.


I found solution: just move the text string from config_extend to config and all is working.

Question is now: which one of the config file is the master ? and why there are 2 files then ?
TIA again

The one file used by phpList is the config.php file.
The config_extended.php file is there to show pretty much every option that can be changed but as it states at the beginning of the file, it is not used unless you change it’s name to config.php and replace the basically “quick start” config.php file.