I wager a guess:
each unit gets assigned a unit ID when it is created and the game probably goes through them in this order - oldest first...
I did not look through the code though, but that is what I would do if I were lazy coding something like this
each unit gets assigned a unit ID when it is created and the game probably goes through them in this order - oldest first...
I did not look through the code though, but that is what I would do if I were lazy coding something like this

) in return for Spice (+Gold +
from Iron etc. is a fluke - it does not exist the way the manual seems to describe it. You just get some extra 

and
are not part of the resource you trade away. They stay on the tile from which the resource is obtained. All other effects
, ability to build Swordsmen from Iron, etc...) are the part you trade away, and having more than one of a certain resource will not give you any more than you already have.