A dark lens and a protective lens are not the same thing, and nothing about looking through a pair of sunglasses reveals which you have. The ratings that describe them measure different properties.
Darkness and ultraviolet filtering are independent
Tint controls how much visible light reaches the eye. Ultraviolet filtering is handled by absorbers built into the lens material or applied as a coating.
A very dark lens with no ultraviolet filtering is worse than no lens at all, because the darkness causes the pupil to open while the invisible wavelengths pass through.
Clear lenses can carry full ultraviolet filtering, which is why some prescription glasses protect as well as sunglasses do without any tint whatsoever.
Category numbers describe visible light transmission
Lenses are graded by how much visible light they let through, from near-clear at the bottom of the scale to very dark at the top.
The scale describes brightness comfort rather than safety, and the numbering conventions and labelling requirements differ between jurisdictions and change over time.
The darkest categories are intended for intense conditions such as snow and open water, and some are marked as unsuitable for driving because they dim signals and instruments too far.
Polarisation solves a different problem entirely
Light reflecting off a flat surface such as water or a road becomes oriented in one direction. A polarising filter blocks that orientation and removes the glare.
This improves contrast dramatically in specific situations and does nothing about ultraviolet, so a polarised lens is not automatically a protective one.
The same filtering makes certain screens appear dark or patterned when viewed at an angle, which is a side effect of the mechanism rather than a defect.
Lens material shapes weight, clarity and durability
Polycarbonate is light and highly impact-resistant, which suits sport and children's frames, but it scratches easily without a hard coating.
Glass gives the best optical clarity and resists scratching well, at the cost of weight and a greater risk of shattering on impact.
Mid-range plastics sit between the two, and much of the price difference between apparently similar lenses reflects the material and coatings rather than the tint.
Mirroring and gradients change comfort, not protection
A mirror coating reflects a share of light before it enters the lens, reducing total transmission and keeping the lens cooler in bright conditions.
A gradient tint is darker at the top and lighter at the bottom, so the sky is dimmed while a dashboard or a book stays legible.
Both are optical conveniences layered on top of the underlying lens. What the lens does about ultraviolet is decided by the material beneath them.