According to the PMBOK® Guide, specifically the Sequence Activities process within Project Schedule Management, there are four types of dependencies used to define the logical relationship between activities.

Discretionary Dependencies: These are established based on knowledge of best practices within a particular application area or some unusual aspect of the project where a specific sequence is desired, even though there may be other acceptable sequences. They are also known as preferred logic, preferential logic, or soft logic.

Application: Project teams typically document discretionary dependencies because they can create arbitrary total float and may limit later scheduling options. During the process of Fast Tracking, these are the first dependencies to be reviewed for potential overlap or removal to shorten the schedule.

Source of Logic: These often come from " lessons learned " or specific technical preferences of the project team rather than a physical or legal requirement.

Comparison with other options:

A. Internal: This involves a precedence relationship between project activities and is generally within the project team ' s control (e.g., a team cannot test a machine until they assemble it).

B. External: This involves a relationship between project activities and non-project activities (e.g., a software project waiting for a government environmental hearing). These are usually outside the project team ' s control.

D. Mandatory: Also known as hard logic or hard dependencies. These are legally or contractually required or inherent in the nature of the work (e.g., you cannot build a roof until the foundation is set). Unlike discretionary logic, these cannot be moved or bypassed easily during schedule compression.