This document is used to provide the reasoning / critical thinking used to effectively communicate your design choices.
Some basic ideas to include could be:
Count and write down how many typefaces and styles used
Recognize the size differences between the type elements
Why are these specific typefaces used
Identify “decorative” or superfluous assets that enhance the design
Identity and describe the influential power of the chosen colour palette
Is there a theme or metaphor used
Go through the layout and number and describe each element in the order of importance
Describe who your primary "audience" is and relate your design choice to preferences of this audience