*** The command C-x RET c (universal-coding-system-argument)
lets you specify a coding system when you read or write a file. This command uses the minibuffer to read a coding system name. After you exit the minibuffer, the specified coding system is used for *the immediately following command*.

So if the immediately following command is a command to read or write a file, it uses the specified coding system for that file.

If the immediately following command does not use the coding system, then C-x RET c ultimately has no effect.

For example, C-x RET c iso-8859-1 RET C-x C-f temp RET visits the file `temp' treating it as ISO Latin-1.