There are two ways this sort of “trade” can’t be made:
One site is already maximally specialized. For instance, if Zion is already fully specialized in growing apples, then there are no further banana or coconut groves to replace with apple trees.
The two sites trade off in exactly the same ratios. For instance, Xenia and Zion both trade off apples:bananas at a ratio of 1:0.5, so we can’t achieve a pareto gain with a little more specialization in those two fruits between those two sites.
If Zion’s fully specialized in growing apples, it can still replace apples with other things.
Note that “multiple goals” might really mean “multiple sub-goals”—e.g. Fruit Co might ultimately want to maximize profit, but producing more apples is a subgoal, producing more bananas is another subgoal, etc.
In that case I think utility is basically linear in each fruit, which makes it a bad example ’cause you don’t need comparative advantage for that. (At least for me, an example of using a concept doesn’t much help remembering it unless it’s helpful to me for that example.) IIUC it’s only useful for subgoals when utility’s sublinear in them.
If Zion’s fully specialized in growing apples, it can still replace apples with other things.
In that case I think utility is basically linear in each fruit, which makes it a bad example ’cause you don’t need comparative advantage for that. (At least for me, an example of using a concept doesn’t much help remembering it unless it’s helpful to me for that example.)
IIUC it’s only useful for subgoals when utility’s sublinear in them.