Not 100% sure I understand exactly what you want but here are some examples of what you can do.
Let's say you want to sum the invoices for customers that have been with the company less than 180 days (let's also assume you don't have a [Length with Company] field or anything like that) but you only want to show the dimension [City]. You could use the following expression:
In the above example you can't use set analysis because the [Join Date] of every customer is going to be different and the set analysis is calculated once per chart, not once per row. Also use the dimension [City] for the example above. You're quesiton was pretty vague but hopefully that helped a little bit.