Sales vs credit sales which one is used to estimate the Allowance for Doubtful Accounts?

Ive seen both sales and credit sales were used interchangeably in this text and in other sources to estimate bad debts. I believe sales and credit sales are distinct in which sales encompasses both cash and credit sales:

Sales = cash sales + credit sales

Now, if we use sales from the equation above to estimate bad debts, weve included cash which is paid already. So, in Sales x Rate = bad debt expense, what which is used as sales?