1.It includes modifications and updations done in order to correct or fix problems, which are either discovered by user or concluded by user error reports.
2. It includes modifications and updations applied to keep the software product up-to date and tuned to the ever changing world of technology and business environment.
3.It includes modifications and updates done in order to keep the software usable over long period of time.
4.It includes modifications and updations to prevent future problems of the software.