@Gaetan I guess I'm not coming across clearly so let me site an example...
Open a game in mame, for example, CONTRA > Make a custom controller input in this game using retroarch then > "save remap file".
The game will save a configuration file in a config file somewhere named "contra.cfg"
Next, open another game in mame for example METAL SLUG. Now this time I want the controls that I've inputed in CONTRA to be the loaded on METAL SLUG so, > "Load remap file" > select "contra.cfg"
I don't know why but I can't load the contra.cfg onto my METAL SLUG controls. I just want to have 1 .cfg file to load to my other game so I don't have to manually change every controls on every game. I know this is possible, I just don't know how.