This excerpt from the text for that section answers your question:

Note: While this part of this exercise is about drawing everything observationally, this is as a step towards understanding the nature of the forms that are present, and how they relate to their surroundings. Later, when we get into drawing the texture gradient, we will be focusing instead on using that understanding in order to invent our own cast shadows for those textural forms, based on their relationship with whatever light source we put into place.

Basically these first two sections of the exercise (the leftmost square and the notes) are where we simply observe and study the reference in order to understand how its textural forms are arranged and how they're shaped, so that we can then use that information to better understand how to design our cast shadows when constructing the texture gradient.