Vulerability of a pair determines the scoring of a contract. A contract by a vulnerable pair usually scores higher, but also is penalised larger when it goes down. Vulnerability plays a major role in competitive auctions.
When a pair has made a game, they become vulnerable. Otherwise, they are non-vulnerable.
The vulnerability of each pair is pre-determined by the board number. NS is vulnerable in boards 2, 4, 5, 7, 10, 12, 13, 15 and repeats for each 16 boards. EW is vulnerable in boards 3, 4, 6, 7, 9, 10, 13, 16 and repeats for each 16 boards.