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These requirements are necessary and sufficient conditions.

Let us understand equation (2.1) for different type of force systems.

1. Collinear forces: In this system, line of action of all the forces act along the same line. For example, consider a rope being pulled by two players. Suppose one player is pulling with a force F1 and the other is pulling with a force F2 . Then the resultant force, in the direction of pull of first player is (F1 - F2). By the first condition of (2.1), it should be zero. Hence, for equilibrium F1=F2 . With this, the second condition will also be fully satisfied, because moments of the forces about any point in the space will be of equal and opposite magnitudes. Hence, the resultant moment will be zero.
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Equation of equilibrium

When a body is in equilibrium, the resultant of all forces acting on it is zero. Thus that resultant force R and the resultant couple M R are both zero, and we have the equilibrium equations

..............(2.1)