From TRM, we know how much requirments coverd in testcase.
by using TRM easily mapping between requirements and 
testcase.

it's very useful while change request time also

A document or mechanism which maps all delivery books of 
each SDLC phase with each other. It facilitates change 
requirement and project tracking.

we write tracebility matrix for understanding the SDLC and 
prepare test senario to write test cases

A RTM(Requirement Treacibility matrix) is a matrix
(generally maintained in an excel sheet) that maps the 
usecases in BRD, technical spec, functional spec and the 
testcases we have written..
When this is done, it would be easy for us to determine the 
gaps...
it helps for gap analysis

The use of Traceablity  matrix is to map the requirements 
with the test cases whether the particular test case is 
performed or not
Muralidharan 
dharanmu@yahoo.com

it is nothing but mapping ur testcase with requirements...
so it is easy to check it out any requirements was
missed.lik that

TR is mapping between requirments and reviewed test cases.
it is useful for to trace the missing test cases

RTM is useful for mapping between the requirements and test 
cases. and also useful for each and every phases of testing 
activities.

Among from the we can refer all the requirments are covered.

Before writing a test cases we can prepare TRM.Why because
after writing a test cases map the TRM and Test cases we can
easily find out the missing functionalities.

TRM is the short for traceability matrix.
this document is able to maintain the modifications of the 
customer.