What does "µ" (mu) mean in plastic thickness?

The designation µ (micron) indicates the thickness of plastic.

1 µ = 0.001 mm
The higher the number of microns, the thicker and stronger the material.

Which thickness do I need?
30 µ → thin film (light products)
50 µ → standard quality
70–100 µ → strong (e.g. carrier bags)
100+ µ → extra strong

Which one should I choose?
Light products → lower µ
Heavy or fragile products → higher µ