The MR=MC rule, or Marginal Revenue equals Marginal Cost rule, primarily applies to only to purely competitive firms.
In a purely competitive market, firms are price takers, meaning they accept the market price set by supply and demand. In this scenario, firms will continue to produce and sell their products until the point where the additional revenue from selling one more unit of output (marginal revenue) equals the additional cost of producing that unit (marginal cost). This is the optimal output level for maximizing profit.
For monopolies and other market structures, the situation is different. Monopolies have the power to set prices above the marginal cost, thus the MR=MC rule still applies, but with different dynamics. In such cases, firms do not operate at the same equilibrium as purely competitive firms and have different strategies for maximizing profits.