Finish – Start (FS)
This is the default logical relationship that is created if the nature of the logic is not specified. If a FS link is defined, the start date of the current activity will be one workday after the latest completion date of any of the predecessors defined. If a lag is defined, the start date of the current activity will be the completion date of the latest predecessor + lag. This concept is illustrated in the following figure:
Finish – Finish (FF)
If a FF link is created, the finish date of the current task will be set to the same date as the finish date of the predecessor unless other predecessor logic conflicts with this assignment. i.e. if both FS and FF relationships are defined to activity predecessors, then the logical relationship that is the most restrictive, will drive the start data of the current task.
If a lag is defined, the finish date of the task will be equal to the completion date of the predecessor + lag. If the duration of the successor task is updated, then the start date will be changed to reflect the increased duration. The finish date will not be changed unless the logical relationship results in a conflict with other defined logic for this task.
Start – Start (SS)
If a SS linked is created, the start date of the current task will match the start date of the specified predecessor. If a lag is defined, the start date will be offset the specified number of working days from the start date of the predecessor.