Project Network diagram is one of the many ways to represent project schedule. You can refer to my other articles on Project Shcedule Lead and Lag and FAQ on Lead and Lag to understand them completely. A lead accelerates the Successor activity whereas a Lag delays it. They just change the Start and Finish events of the dependent activities. These Modifiers do not change the nature of a logical relationship. PDM supports two Modifiers, which can applied to each relationship. Note: Since milestones have zero duration, their Start and Finish events are same. FS means Finish of Predecessor and Start of Successor. In all the above relationships, Predecessor’s event is written first and Successor’s event is written later e.g. You can refer to Start-to-Finish (SF) Relationship to understand it thoroughly.