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Emacs keeps a flag called the “modified flag” for each buffer, to record whether you have changed the text of the buffer. This flag is set to ‘t’ whenever you alter the contents of the buffer, and cleared to ‘nil’ when you save it.
Some Lisp programs set the flag explicitly. For example, the function ‘set-visited-file-name’ sets the flag to ‘t’, because the text does not match the newly-visited file, even if it is unchanged from the file formerly visited.