Briefly describe the three main types of software maintenance. Why is it sometimes difficult to distinguish between them?

What will be an ideal response?

three main types of software maintenance are:
1. Corrective maintenance or fault repair. The changes made to the system are
to repair reported faults which may be program bugs or specification errors
or omissions.
2. Adaptive maintenance or environmental adaptation. Changing the software
to adapt it to changes in its environment e.g. changes to other software
systems.
3. Perfective maintenance or functionality addition. This involves adding new
functionality or features to the system.
They are sometimes difficult to distinguish because the same set of changes
may cover all three types of maintenance. For example, a reported fault in the
system may be repaired by upgrading some other software and then adapting the
system to use this new version (corrective + adaptive). The new software may have
additional functionality and as part of the adaptive maintenance, new features may
be added to take advantage of this.