What Does Arm's Length Transaction Mean?
A transaction in which the buyers and sellers of a product act independently and have no relationship to each other. The concept of an arm's length transaction is to ensure that both parties in the deal are acting in their own self interest and are not subject to any pressure or duress from the other party.

thanks.
you mean the free market transaction,for the arm's transaction.

学到知识了，那么后两种贸易方式作何解释？请赐教，谢谢

得加强英语。。呵呵

i don't know why  this kind of division u have mentioned can be drawn
actually, in the textbook of international economics, there are :
1  interindustry trade, which is due to the theory of comparative advantage
2  intraindustry trade, which is a result of scale of  economics while based on comparative advantage
3  division of labor and specialization furthers into the product level, which we call intra-product trade