Some of the data points that the issuer gets is your total debt, total credit limit with futher segments for auto loans, mortgages and of course credit cards. If you are like most Americans and pay your credit card bill once its generated the following will happen.

Issuer A sends bill for $300 to clear entire balance or $20 minimum payment
You pay in full $300 bringing the account to $0
Issuer A based on the end of month billing sends to the bureau that your account had a $300 balance
Note: not every issuer has to work this way and may choose to report differently

Even though you paid the debt in full, the bureau will show the balance as of the last month. Now to the question, how can the issuer determine whether they are “top of wallet”. This becomes more of a judgemental call, but is fairly easy to calculate.

If the issuer takes your total credit card balance at the bureau (including its own) – it will sum all credit card debt both transacting – PIF every month and revolving – balance with an interest. If your current credit card debt number at the bureau is $3000 and the issuer sees they only have $300 of that balance from your last statement, its fairly safe assumption that they are at the bottom or near the bottom of the wallet. Now if you repeat the same calculation and the issuer sees $1500 of $3000 at the bureau – now the issuer can safely assume that this card represents is the or near your primary card.

While not exact and each issuer has different criteria this represents the easiest way to determine whether a card is a consumer’s primary.