Whenver there is a modification in existing functionality,
it has to be tested again which is called regression 
Testing.
for regression Testing we use 
2 documents:
Design Document
Traceability metrics

Regression Testing means checking new or added 
functionalities effect on the existing system components. 
So that we can check changes are correct and it doesnt 
adversely affect the existing system.

This can be done using Batch Test using Call statements.

Hope this is enough for your query.

Regression testing is to test whether changed functinality 
has not regressed,impacted anything in the existing 
functionality

Approach for regression testing:

1. Carry out the impact analysis (Change and the impacted 
areas around the change)

2. Select all the major test cases with the maximum 
functional coverage. You may draw a traceability matrix to 
identify the coverage.

3. Prepare a regression pack with the maximum test 
coverage. 100% regression testing is ideally not possible. 
Focus on the major functionality and impacted areas around 
the change.

4. carry out the regression testing

regression testing comes to picture, when any changes or 
modification in existing build or application.In this 
testing we test the new changes or new modification & check 
that there is any changes in exist system due to changes.
for example when a raise a bug then he assign to respective 
developer, then developer work in coding & release new 
build to testing department.then tester test the new build, 
and also check there is any impact on exist system due to 
new changes.