There are two common ways of classifying accounts. One way, which is what the questioner is using, is to classify by the total volume of sales and/or gross profits. Thus, an A customer is someone who bought more than a B customer. The other way, which I believe is more effective for sales people, is to classify the customer based on potential. Thus, an A customer has the potential to buy more than a B customer, regardless of what they have actually purchased.