In the project management world, dependencies determine the order in which various activities should be completed. These dependencies usually documented as an activity attribute and help in sequencing the activities on the project schedule network diagram. As you prepare for your PMP exam, it is important to know the difference between discretionary dependencies and mandatory dependencies.
A mandatory dependency is one that “must be” carried out at a particular time. It is usually requirement of some kind based on contracts, laws, company procedures, physical limitations, etc. When the sequence of events is developed for various aspects of the process, mandatory dependencies are placed where they must happen.
A discretionary dependency is one that isn't based on a "have to", but on a "should". These decisions are usually based upon best practices, business knowledge, etc. They are placed on the project diagram where the team members would like them to occur.
Since discretionary dependencies are more arbitrary, they should be fully documented so the reasons for their placement can be maintained and available during future revisions. As a project progresses and adjustments are needed, discretionary dependencies are often reviewed for possible alterations.