Activities and Delays
-An activity is a period of time whose length is known before the activity begins. As a result, the duration's start and end dates can be scheduled. The duration can be a constant, a random value from a statistical distribution, the result of an equation, file input, or computed based on the event state.
-A delay is an indefinite period of time caused by a combination of system conditions. When an entity joins a queue for a resource, the time it will remain in the queue is initially unknown because it depends on other events that may occur.
