In the text, the "red$-$line method" refers to

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

Restrictions imposed by companies that insure credit risks.

The use in Dun & Bradstreet's reports of a red line to show the maximum amount of credit that 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."

A method of controlling inventories by drawing a red line on the inside of an inventory storage bin and ordering more of the item when the stock falls below the red line.

A method of controlling receivables by drawing a red line on the credit reports of companies that are likely to pay latflmpr actually default.