Existence Of Activities
Using conventional Project Management tools, as soon as an activity is described, it exists and must be carried out, even if delayed. If you are putting up a high-rise, it is not an option to leave out the 23rd floor. When you are planning a number of interrelated projects, some of which will overrun or supplant others, it is necessary to have the higher level plan include activities (projects) whose existence can be controlled. Waves of technology is just one example, where delaying one wave will cause it to be overrun and supplanted by another, so the activities relevant to the wave should be terminated if in progress, or cease to exist if not started.
Changing the duration of an activity to zero is not sufficient to eliminate the effect of the activity in the network, as predecessor/successor relations that related the activity to other activities in the network would still be operational. If the activity no longer exists, any predecessor/successor relations acting through the activity must also cease to exist, or at least have no influence on other activities.
In the diagram, if the modelling is correct and Activity_2 ceases to exist, there will be no predecessor to Activity_3, allowing it to start on day 1.
ORION provides a logical control pin on all activities. For most activities, where there is no intention of controlling existence, this pin is unconditionally set True, the state only being changed when it is desired to lift the constraints during mass updating.
Setting this pin to False for an activity has the effect of disabling the duration connection between Start Date and Finish Date, so both the Start Date and Finish Date are only controlled by external influences and may range over the entire duration of the calendar.
An activity may monitor its existence. If the logical control pin is initially in an indeterminate state (neither True nor False), the activity monitors the values on its Start Date and Finish Date pins. If the duration can no longer be fitted between the minimum Start Date and maximum Finish Date, the activity will broadcast a False out of its control pin, effectively terminating its own existence. The same effect may be achieved with cost, risk or any other resource, an activity turning itself off if there is insufficient resource availability.
In the same way that all constraints are logically controllable, the addition of a control pin to an activity permits logical control of activities, needed for Program Management of multiple projects and planning of single projects where alternatives, backups or contingencies exist.
There is a more detailed version of existence used for relations - see Existence Presentation