In a reinforced concrete slab, the amount of reinforcement for main bars is designed to resist bending (flexural) action.

Explanation
Main reinforcement bars are provided in the direction of span, where the slab experiences tensile stresses due to bending under applied loads.

Concrete is weak in tension; therefore, the main bars are designed to carry the tensile force resulting from flexural moments.

The area of steel is calculated based on:
- Design bending moment (due to dead load, live load, etc.)
- Strength of materials (concrete and steel)
- Limit state of collapse in flexure

Clarification:
The Main reinforcement is resist flexural tension due to Maximum bending moment, and
The secondary (distribution) reinforcement is for temperature and shrinkage stresses resistance.

Therefore, the amount of reinforcement for main bars in a slab is designed to resist flexural tensile stresses due to maximum bending moments.

In slab design, the maximum design bending moment (that could be positive or negative, depending on support conditions), governs the required area of tensile steel.

The reinforcement is calculated such that it can safely resist the worst (Maximum) bending moment produced by the governing load combination.

Although minimum reinforcement is also specified by design codes, it is a separate requirement intended for crack control and ductility treatment, not for resisting bending moments.

In concluding, take these two points as takeaways.
I) Main reinforcement is Designed for maximum bending moment, and
II) Minimum reinforcement, that is Code mandated lower limit for serviceability and crack control.

Hence, Main bars in a slab are designed based on the maximum bending (flexural) moment.