In our app we chose to simplify our use of these terminologies so that it'd be easier to understand.
An up-sell is when you get a customer to swap a product to increase the value of their cart size.
A Cross-sell Is when a customer adds an additional product into their cart alongside the.
A down-sell is when you offer a customer a cheaper alternative/when a customer rejects an offer that you've shown them in your funnel.
A trigger is what causes your popup/funnel to appear.
Your Funnel Flow is the overview and outline of what your customer's experience depending on how they react to each pop up.