What is the 50 rule in real estate?

Could you please elaborate on the concept of the "50 rule" in real estate? I'm curious to understand how it's applied and what it signifies in terms of property investment and management. Is it a rule of thumb for determining profitability, or does it relate to a specific aspect of real estate transactions? I'd appreciate it if you could provide a concise yet informative explanation of this rule and its significance in the industry.