Tracibility means each requirement is mapped to one or several testcases...or one test case is mapped to many requirements... if you can go futher and from requirement and trace to end product then it is said forward tracibility..& and if you can move from end product to the requirement then backward tracibility...and in a single matrix if you can move in both forward and backward direction then it is said Bi directional matrix.
Myself - I've never made a distinction of bi-directional versus forward or backward traceability. I would think that by definition, since you are relating two items in the matrix, it would always be bi-directional. But I may be wrong.
yes yr rt that v can relate as many things in matrix to refer things... but either movin forward or backward... the saying that moving in both ways (back & front) is bi- directional matrix...and their are cases where with the help of defect Number one cannt travel to the requirement.