Bi-directional Traceability Question

Would you please explain to me the meaning of bi-directional traceability? If I am not wrong, Horizontal Traceability indicates mapping of a requirement across all the SDLC phases like Requirement->Design->Construction->Testing. But I do not get what is meant by Vertical Traceability. Would you please educate me on this?

Vertical traceability is top-down bottom-up traceability. Vertical traceability is the most common type of traceability. Basically it is tracing from the highest level requirements all the way down to the product component level (could be even to a specific line of code). And then starting at the lowest level and tracing all the way back up to the top. Horizontal traceability is tracing within a document or between peer documents. What you have listed as tracing from requirements to design to construction to test is actually vertical traceability. And when you come right down to it, it doesn’t really matter what type of traceability you label it. All that matters is that you can trace forward from a starting point to an end point and vice versa. The level and complexity of the traceability is determined by the criticality of the product and project. For a program like the Space Shuttle or Space Station, you would need very rigorous requirements traceability. Whereas a small application or a product would only need some simple traceability.