ما هي القاعدة العشرين في الأسهم؟

Could you elaborate on the concept of the "20 rule" in the context of stock investments? I've heard this term mentioned in relation to financial strategies but I'm not entirely clear on its specifics. Is it a guideline for allocation, a risk management principle, or perhaps something else? Understanding the finer details and the logic behind it could potentially aid in making more informed decisions when it comes to managing my portfolio. Clarifying its meaning and applications would be greatly appreciated.