An instrument noun denotes a tool or means used to perform an action. The book gives templates including مِفْعَلٌ، مِفْعَالٌ، مِفْعَلَةٌ and notes less common alternatives.
PatternExampleFunction
مِفْعَالٌمِفْتَاحٌKey, used for opening
مِفْعَلٌمِبْرَدٌFile, used for filing
مِفْعَلَةٌمِكْنَسَةٌSweeping implement
فِعَالٌخِيَاطٌSewing-related implement, in a specialized use
Both the word's pattern and its real lexical function matter; a prefixed mīm by itself does not prove an instrumental meaning.