Economic Union combines the effects of Town Charters and Naval Infrastructure, doubling the adjacency bonuses of Commercial Hubs and Harbors. Considering that it becomes available around the same time as the much more useful Market Economy, it's hard to justify using an Economic slot on it.
Under the impetus of globalization, smaller nations have found themselves unable to compete with the larger states in terms of capital, labor, production, and raw materials. To that end, a number of economic unions – trade blocs composed of a common market and a customs union, often with some sort of unifying currency – began to coalesce in the 1980s. Member nations agree to common product and business laws, free cross-border movement of goods and services, joint external trade regulations, and so forth. A number have been cobbled together over the past few decades: CARICOM, the Eurasian Economic Union, Mercosur, but undoubtedly the biggest and most influential is the European Union created by the Maastricht Treaty in 1992 AD.