This tier is essentially everything else. These types of sources—especially Wikipedia—can be helpful in identifying interesting topics, positions within a debate, keywords to search, and, sometimes, higher-tier sources on the topic. They often play a critically important role in the early part of the research process, but they generally aren’t (and shouldn’t be) cited in the final paper.
I found this section by the author pretty interesting. I believe that this was interesting because the author talked about sources that we could possibly get information from but aren't good enough to put in a final paper. I personally have come across many sources like this. I believe you can still get valuable information from these type of sources but you should not rely on them.