Deepest distinguishable shadow value on film stock — defines black point. Overexposure kills it permanently, no recovery in grading.
When shooting on film, maximum density is your lowest anchor for grading. It's not simply "black" — it's the darkest tonal zone on the negative that remains photochemically distinguishable before the emulsion is completely pushed to its limit. On set, this means your blacks have a boundary, and it's not at zero.
In practice, the most important thing happens during exposure. If your negative receives too much light — overexposure — the maximum density collapses. The darkest areas lose their tonal differentiation and become an undifferentiated, flat blackness. This is irreversible. You can't recover it in the lab. That's why many cinematographers work conservatively: better to underexpose slightly than overexpose, because the blacks are your last controllable element. With underexposure, you still have material to recover in the highlights, but with maximum density, it's over.
The exact density depends on: film stock (Kodak Vision3, Fujifilm Eterna — each has its peculiarities), development (time, temperature, chemicals), and light wavelength. This is measured during test developments using a densitometer or step wedge. Typical values range between 3.0 and 3.5 density units — but that's territory for lab specialists. On set, what interests you most is: expose correctly, keep the shadows open, and trust your light meter.
It becomes particularly critical in low-key or night scenes, where a lot happens in the dark. There, you quickly lose detail in the blacks if you don't bring at least a hint of fill light — be it through practical lights, fill light, or reflected surfaces. This isn't an aesthetic toy, but a technical necessity to utilize maximum density meaningfully, rather than wasting it.