Bidirectional Traceability of Software Requirements

Requirement Traceability - Bidirectional traceability, what is Horizontal traceability; What is vertical traceability; Why is this required?.......... Similar questions are galore for any QA Engineer. See if this clarifies these doubts:

Following is the 'text' as is, on the terms 'Horizontal traceability' & 'Vertical traceability'
from different sources.

(1) Source: http://www.sei.cmu.edu/cmmi/faq/new-faq.html

In the Requirements Management (REQM) process area, specific practice
1.4 states, "Maintain bidirectional traceability among the requirements and the project plans and work products." Bidirectional traceability primarily applies to vertical traceability and at a minimum needs to be implemented both forward and backward (i.e., from requirements to end products and from end product back to requirements).

Vertical traceability identifies the origin of items (e.g., customer
needs) and follows these same items as they travel through the hierarchy of the Work Breakdown Structure to the project teams and eventually to the customer. 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.
Horizontal traceability is also important and is mentioned in subpractice 3, but it is not required to satisfy bidirectional traceability.

Horizontal traceability identifies the relationships among related items across work groups or product components for the purpose of avoiding potential conflicts. It enables the project to anticipate potential problems (and mitigate or solve them) before integration testing. For example, horizontal traceability would follow related requirements across two work groups working on two associated components of a product. The traceability across these two work groups enables the work groups to see when and how a change in a requirement for one of the components may affect the other component.
Thus, horizontal traceability enables the project to anticipate potential problems (and mitigate or solve them) before integration testing.
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(2) Source:

Author: Tim Kasse
Title: Practical Insight into CMMI
Pg 153-154

Vertical traceability: Traceability `from requirements through the associated life-cycle work products of architecture specifications, detailed designs, Code, unit test plans, integration test plans, system test plans, so forth and back"

Horizontal traceability: refers to the traceability from the requirements to the associated plans such as the project plan, quality assurance plan, configuration management plan, risk management plan, and so forth
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(3) Source:

Author: Shari Lawrence Pfleeger
Title: Software Engineering - Theory & practice
Pages: 439-441

Vertical traceability expresses the relationship among parts of the workproduct. For example, vertical traceability of the requirements describes the interdependencies among the system requirements.

Horizontal traceability addresses the relationship of the components across collections of workproducts. For instance, each design component is traced code components that implements that part of the design