The question is asking for the category with most refunds in terms of their count (total refunds), not the ratio of refunds to sales.
The term "last month" is taken to mean the last complete calendar month (Sep 2023).
By "biggest cities" we mean those with 100+ ticket sales.
The query counts the number of unique events, not the total number of events.
The query only considers returns with "returned_at" date occurring within the last 30 days
The query does not filter out any specific products, so the results include all products present in the data.