I'll be honest mate, you'll never get a straight answer here since it's far too vague and subjective a question.
Players in the AMC position are what I refer to as "linking" players. They act as a link between lines and provide an extra dynamic
when attacking since they operate between the midfield and defense. They themselves are not effective unless the midfield and forward line is utilizing them correctly. What roles, duties and instructions are completely dependent on how you are trying too set the team up.