What is Order dependency in class SDep

LLVM Project
1

In below dependency kind in file ScheduleDAG.h, we could see WAR, RAW,WAW, not see RAR. So dose Order dependency is RAR or anything else? Can we have a example for it.

    enum Kind {
      Data,        ///< Regular data dependence (aka true-dependence).
      Anti,        ///< A register anti-dependence (aka WAR).
      Output,      ///< A register output-dependence (aka WAW).
      Order        ///< Any other ordering dependency.
    };