What is "When signing up to a social networking site, the username field must be filled in"?
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What is "Bar codes or ISBN numbers will contain an extra number – a check digit – at the end. By looking at this, it will know if the longer set of numbers is incorrect"?
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What is "Shopping sites may restrict postcodes to the following combination of letters (L) and numbers (0): LL00 0LL"?
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What is "data by entering it twice and checking both are the same"?
