Hi,
Your question about the relationship between familiar/novel, exposure level, and session_type should be addressed by the response to another forum post here.
Another important thing to note is that the eLife paper you linked did not use the same dataset as the Visual Behavior 2P project dataset you are using as part of the Neuromatch course. The eLife paper used a pilot dataset that we collected prior to launching large-scale data collection for the Visual Behavior project.
We used these pilot experiments to get initial results and refine our experimental design for the larger dataset. For example, in the Visual Behavior dataset, we only use 1 novel image set, but we include multiple sessions with that image set to allow tracking of changes in neural activity as novel images becoming increasingly familiar across multiple exposures, whereas the eLife paper only includes one session for each novel image set.
So you are correct that image sets C and D are not available in the dataset you are working with.
Please let me know if you have additional questions.