You want to add a comment to March 2020 as shown in the following visualization.

To combine two sets, they must have at least one dimension in common. In this case, Customers of 2020 and Top N Customers both have Customer Name as a dimension, so they can be combined using set operations such as union, intersection, or difference. Sellers of 2020 and Top N Products have different dimensions (Seller Name and Product Name), so they cannot be combined. References: https://help.tableau.com/current/pro/desktop/en-us/sets.htm https://help.tableau.com/current/pro/desktop/en-us/sets_create.htm#combine-sets

In Tableau, sets can be combined if they are created from the same dimension. From the options provided, "Customers of 2020" and "Top N Customers" are likely created from the same dimension (Customers). Therefore, these two sets can be combined to create a new set that includes or excludes members based on the combined criteria. Combining sets like "Top N Products" with "Customers of 2020" would not be feasible unless they are from the same dimension, which is typically not the case.