Some say a design is never really completed. In fact most designs are actually improvements or changes from existing designs. When your design is finished you will invariably see deficiencies or areas of improvement which if given enough time you would like to fix up. In real life there is never enough time to address all of these concerns. At this point you may reflect on the design process, things which you think went very well or which may need improvement if you were to do it again.
In the reflection section you may document your reflections about the project.
You will most definitely want to write down your ideas for improvement as if another design team was to take on your project for the next generation of the design. There is nothing wrong with being self critical about your own work especially if you include suggestions or ideas for improvement.