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Using SWUs in region 2 (outside North Amercia) require a "W" fare or higher.

From http://www.united.com/page/article/0...d%20Conditions

Upgradeable Booking Classes fares: This upgrade may be used on United Business booking classes (C, D) and United Economy booking classes (Y, B, E, M, U, H, Q, V, W, S, T, K, L) for travel in Region 1. This upgrade may be used on United Business booking classes (C, D) and United Economy booking classes fares (Y, B, E, M, U, H, Q, V, W) for travel in Region 2.