Studied economics.

One is static (equilibrium state) and the other is dynamic.

When MC equals ATC, the competitive market is in equilibrium. At this time, the price, output, demand and cost are all in balance, and nothing has changed. This is static equilibrium. It is difficult to achieve in reality.

The reality is that both cost and market demand are changing with time. Accordingly, the output and price are also changing. If MC is less than ATC at a certain moment, the enterprise will lose money and must reduce production or face bankruptcy. Therefore, the market will soon adjust to make MC greater than ATC, so any situation that MC is less than ATC can only be temporary. In the long run, MC must be greater than ATC. And MC is equal to the static equilibrium point of ATC, which is hard to see in reality.