Create a parameter with two values 'Value' and 'Volume'.
Then create a calculated field as
WHEN 'Volume' then SUM(Quantity)
WHEN 'Value' then SUM(Revenue)
A conditional probability is the probability of an event, given some other event has already occurred. In the below example, there are two possible events that can occur. A ball falling could either hit the red shelf (we'll call this event A) or hit the blue shelf (we'll call this event B) or both.