The brightest area in frame that still retains detail — nail your white reference to lock exposure and color balance. Steer it directly into the histogram without blowing highlights.
On set, it always works the same way: you point your camera at a gray card or reflector, measure the brightest point in your scene, and decide where this point should land in the digital signal. That's your White Point — not absolute white, but the highest brightness that still retains detail, still has information. You avoid clipping by consciously staying below the maximum sensor value. This sounds theoretical, but it's crucial for balancing: if you set the White Point incorrectly, the entire exposure will be pulled in the wrong direction.
Practically, it works like this — you look at the histogram, not the monitor. The monitor lies with backlight, with reflections, when the sun is in an unfavorable position. The histogram shows you the actual data stream. You need headroom at the top: at least 5-10% reserve between your target White Point and the absolute maximum. With Log recording (like with Arri Alexa or Red), you're working in a compressed curve anyway — there, you set the White Point at about 90% of the signal to have room to maneuver later in color correction. With standard gamma, there's less room.
Color plays just as much of a role as brightness. If your White Point is too warm, you'll get a systematic yellow or orange cast throughout the entire image — especially visible in the highlights. With white balance (manual via gray card or automatic), you correct this before the camera rolls. Some DoPs intentionally set the White Point slightly warm to give more warmth to an overexposed daylight setup; that's a creative choice. But it must be conscious, not a result of measurement error.
In the edit, it becomes critical: if your White Point was too hot, the highlights are blown out, and you can't recover them. If it was set too cool, too dark, then the entire image section looks underexposed, flat, without radiance. The White Point is therefore not just a technicality — it's the anchor point for exposure and color temperature simultaneously. Most importantly: measure it under the conditions under which you will be shooting. Don't look at an indoor test shot if the scene is taking place outdoors.