The intent of this specific practice is to maintain the bidirectional traceability of requirements for each level of product decomposition. When the requirements are managed well, traceability can be established from the source requirement to its lower level requirements and from the lower level requirements back to their source. Such bidirectional traceability helps determine that all source requirements have been completely addressed and that all lower level requirements can be traced to a valid source. Requirements traceability can also cover the relationships to other entities such as intermediate and final work products, changes in design documentation, test plans, and work tasks. The traceability should cover both the horizontal and vertical relationships, such as across interfaces. Traceability is particularly needed in conducting the impact assessment of requirements changes on the project plans, activities, and work products.