I work in the office that settles charges at the end of the day. Some times we have trouble with charges not having an authorization code. Those don't go through. But sometimes the authorization codes vary greatly. What so those numbers mean, and how can they vary. Sometimes they have all numbers, or a mixture of numbers and letters. What is the rule here. Can they have all letters and no numbers?

Why would you need to know this? The card issuer generates their codes. What the codes mean and how they are generated is proprietary info.