'Xs' must have predecessors and successors, but must not be aligned through leads or lags.  Alternately, 'Ys' must always have leads and lags, as well as having a clear duration and resource requirements.

'Xs' are elements of work normally found in a WBS, which have an expected duration, cost and resource requirements.  'Ys' refer to significant events that normally have no duration, cost or resource requirements associated with them.

'Xs' by definition have no duration, but must have a logical relationship with 'Ys'.  These relationships may include the provision of leads and lags.  Alternately, 'Ys' must have a duration and resource allocations, so they can be applied to coordinate the Xs.

'Xs' refer to significant events that normally have no duration, cost or resource requirements associated with them.  'Ys' are elements of work normally found in a WBS, which have an expected duration, cost and resource requirements.