Trade-in (any device, for specific device)

Trade-in is a return value for a used device (but not too old). E.g. an iPhone XR is currently valued for trade-in at “up to USD 90”, so up to just 10% of its original value. That “up to” implies that the appraised value might be less. That value can then be used against any Apple hardware purchase; it doesn’t have to be the same type. Two devices traded in would be like using two gift cards, likely plus some additional cash, to buy a new item there.

As your fellow end users, what we know is in this Apple Trade-in page:

Apple Trade In - Apple

Be sure to check the FAQs and well as the fine-print footnotes.

If you need something more official, you'll need to ask Apple and they will not see your question here.