It is an error if required events are not detected. In addition to events, there are also constraints. Constraints persist over certain intervals of the property. A constraint, denoted as a horizontal line, starts at one mark, and ends at the same or another mark. A constraint may or may not include the mark to which its end point is assigned.
Marks are the blue, vertical lines on which events are placed.
Each mark can only have one event. Marks are not clock ticks, but rather are event ticks. If event e1 is assigned to mark 1, and event e2 is assigned to mark 2, we know nothing about the interval of time between events e1 and e2. We do know that that e1 precedes e2 and that there are no other events of interest to our property that occur between e1 and e2. There may be other events between e1 and e2 that are not of interest to our property, but we do not know how many of these events, if any, occur between the occurance of e1 and e2.