_{I am non native english speaker but I'm willing to improve.}

• Don't overwrite file. If you just want to add new things.
  ~Don't use a file name that is the same with original file.

  example: Japan Ship name mods
  when create new file at "MOD_FOLDER_NAME"\common\units\names_ships\
  Use name like: "MyMod"_ship_names
  Not : JAP_ship_names
  
  then in MyMod_ship_name write
  for_countries = { JAP }
  if you want to use with only japan.

  
  
• still not sure how the file work. It feel like the game read whatever in those folder regardless the tag name

  example: MyMod_ship_name -work. It not neccessary to be like "COUNTRY TAG"_ship_name (i.e. JAP_ship_names)

  
  
• the Uppercase code is likely to be used on some other files.

  example: JAP_SS_HISTORICAL = { name = NAME_THEME_HISTORICAL_SUBMARINES ... } NAME_THEME_HISTORICAL_SUBMARINES used on localisation files