You should know about the break-even ratio (BER, also sometimes called the default ratio) because it is a benchmark often used by lenders when underwriting commercial mortgages. Its purpose is to estimate how vulnerable a property is to defaulting on its debt should rental income decline. There is an old saying that when your outgo exceeds your income, your upkeep will be your downfall. Essentially, the lender is trying to gauge the proportion between your outgo and your income, so as not to share in that downfall.
You should review debt coverage ratio (another major benchmark favored by lenders) in the previous chapter and loan-to-value ratio in Part II, Calculation 26.