Ted, your explanation is clear to me.

For here the "set" is a folder, and the member is "document", so the "sum" of all the members can mean nothing but the number of documents in the cluster.

If the case is a general set, which might be a set containing numbers, say 1,2 and 3, "the sum of all the members" then can mean 1+2+3=6, or the total count of all the members, i.e., 1+1+1=3. In this case, should we avoid use the word "sum"?

Do inform me if you see any corrections needed in my written English.