I'm not totally sure about your goal, but here are a few comments about your questions:
- Variable can only hold a single value. So, if you need a single number, you can calculate and store it in a variable, but not if you need to store multiple values.
- On the other hand, you can create conditional aggregation in a variety of ways (Set Analysis being the best for performance) and limit the aggregated values to specific values of some dimensions (if that's what you are trying to accomplish)
- Table Box is not the best object to use, for a variety of reasons. It's highly recommended to use Straight Table instead. You will have more flexibility and less performance issues this way.