Schedule Quality
Hard Constraint
A hard constraint pins an activity to a fixed date -- Mandatory Start or Mandatory Finish -- overriding the logic-driven dates the schedule would otherwise calculate.
Definition
Most activity dates in a healthy schedule are calculated dynamically from logic: an activity's dates shift automatically as its predecessors' dates change. A hard constraint breaks that -- Mandatory Start and Mandatory Finish pin an activity to a fixed date no matter what its predecessors do, which means the constrained activity won't move even if everything feeding into it slips. That's what makes hard constraints one of the more dangerous DCMA findings rather than just a data-quality nitpick: a real delay upstream can be silently absorbed by a hard-constrained activity downstream, with the schedule's dates holding steady and giving no visible signal that anything went wrong. (Softer constraint types -- Start-On, Finish-On -- are more forgiving and aren't flagged the same way, since they still let the schedule react to real dates rather than fully overriding them.) A schedule leaning on hard constraints to hit its finish date is one that deserves a second look before its dates are trusted at face value.
Example
An activity 10 steps upstream slips two weeks, but a downstream milestone carries a Mandatory Finish constraint and shows no change in the next update -- not because the delay didn't happen, but because the constraint prevented the schedule from reflecting it.