The concept of a logical relationship in reference to the project management scheme refers specifically to an established and existing reliance or dependency between two particular elements of the project. Some examples of these types of relationships include the relationship that can exists between a selection, pair, or grouping of project work schedule activities, as well as the relationships that can exist between a particular project schedule activity and a unique schedule milestone. For more information on the concept of these types of relationships, please review the definition for the term precedence relationship. There are, in fact, 4 specific types of logical relationships that should be made a note of. These 4 types of relationships are: Start to finish relationships, Start to start relationships, Finish to start relationships, and finish to finish relationships. Logical relationships can exist in a standalone capacity, or they can exist on conjunction with other logical relationships.
This term is defined in the 3rd and the 4th edition of the PMBOK.