Like everything, it's vague and poorly written. The basic gist is that it's there to help uphold/protect the Constitution. The power and discretion belongs completely to the president, but it's there to assist the authorities already in place. Supposedly, if the president goes rogue with it, then the courts can intervene(after the fact and ineffective like everything). General civil disorder would be the national guard, unless it's martial law, where the federal government takes over everything.