In a software development process, a traceability matrix is 
a table that correlates any two baselined documents that 
require a many to many relationship to determine the 
completeness of the relationship. It is often used with 
high-level requirements (sometimes known as marketing 
requirements) and detailed requirements of the software 
product to the matching parts of high-level design, 
detailed design, test plan, and test cases.

Common usage is to take the identifier for each of the 
items of one document and place them in the left column. 
The identifiers for the other document are placed across 
the top row. When an item in the left column is related to 
an item across the top, a mark is placed in the 
intersecting cell. The number of relationships are added up 
for each row and each column. This value indicates the 
mapping of the two items. Zero values indicate that no 
relationship exists. It must be determined if one must be 
made. Large values imply that the item is too complex and 
should be simplified.

To ease the creation of traceability matrices, it is 
advisable to add the relationships to the source documents 
for both backward traceability and forward traceability. In 
other words, when an item is changed in one baselined 
document, it's easy to see what needs to be changed in the 
other.