External differentiation refers to the various relationships the EU has with neighbouring countries with which it has a formal agreement. There is a general principle that the more access associated countries have to the EU Single Market, the more regulatory alignment they must accept. There is, however, no standard formula for achieving this. The only standardised arrangement is the European Economic Area and even that varies in detail for the individual states. Otherwise the arrangements differ according to concerns about sovereignty, sectoral interests, regulatory styles, power politics and the salience of particular issues and the overall trajectory of associated states towards or away from the EU.