Defines how one [[activity]] is related to another. A relationship can be one of four types and may include a [[lag]] time:
1. [[Finish to start]] (FS) activity cannot start until another is finished.
2. [[Finish to finish]] (FF) activity cannot finish until another is finished.
3. [[Start to finish]] (SF) activity cannot finish until another begins.
4. [[Start to start]] (SS) activity cannot begin until another starts