Vertical traceability identifies the origin of items 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.

Horizontal traceability is not required to statisfy bidirectional traceability.It identifies the realatinships among related items across work groups or product components for the purpose of avoiding potential conflicts.

I guess it tells us about the traceability matrix. In RTM we have forward and backward traceability. This is done to ensure that the requirements/functionalities as in the Specification are all documented as test case.

Forward traceability:

Tracing a particular functionality with the testcases associated to it

Backward traceability:

Identifying the testcases associated with a particular functionality

.

Vertical Treceability: The relationship among the parts of a single workproduct (discipline)

• e.g. requirements.

Horizontal Treceability: The relationship of the collections of components across collections of workproducts

• e.g. a design component is traced to the code components that implement that part of the design.

Regards

Prasad