In the text (wiki this if it is no longer in the text), the "red-line method" refers to

a. The policy of drawing a red line around certain neighborhoods on a map and then refusing to sell on credit to people who live within those areas.

b. Restrictions imposed by companies which insure credit risks.

c. The use, in Dun & Bradstreet's reports, of a red line to show the maximum amount of credit which should be extended to a given customer; companies using this limit when they screen customers' orders are said to be using the "red-line method."

d. A method of controlling inventories by drawing a red line on the inside of a bin.

e. A method of controlling receivables by drawing a red line on invoices of companies that are expected to pay late.