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. Reference: 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