What is the disadvantage of engaging in in strategic trade policies

What is meant by external economies of scale and how does their existence lead to the creation of "industrial district". What are "industrial district" and what are the three main sources of the economic advantages derived from locating in such a district? What is their implication for international trade?

Describe a model whereinternal economies of scaleand differentiated products lead tointra industrytrade. What is the welfare implication of this model