Task B cannot start until Task A finishes (most common).
While early CPM used Arrow Diagramming, modern scheduling relies heavily on PDM. PDM allows for more complex, realistic relationships between activities than simple finish-to-start logic: construction planning and scheduling jimmie hinze pdf
[Activity A] ---> [Activity B (Critical)] ---> [Activity D (Critical)] ---> Project Finish ---> [Activity C (Float)] ---> Task B cannot start until Task A finishes (most common)