A Planning-Constraint associated with a particular Activity or Activity-Spec that is required to be true in order for the successful performance or execution of the Activity or Activity-Spec.It contains two main things: 1) a State-Of-Affairs that is brought about, or made true by the execution of the Activity or Activity-Spec; 2) A specification of when that State-Of-Affairs must be true; this may be one of: to hold immediately after T-Begin, immediately after T-End, or throughout the whole Actual- or Specified-Activity-Interval