Because of iterative development, there is a possibility of multiple releases being used at different places. Upto one old software release is maintained.
After final delivery (acceptance tesing done), a period of 1 month is observed, where any residual faults are fixed free of costs. Post this period, such problems are fixed as part of Annual Maintenance Contract.
AMC costs are based on the scope of work required in AMC. This is of following types: a. Only fixes to faults observed b. Minor enhancements, DB updates c. Rate Contract for feature implementations