Alpha channel separate from RGB — color values not premultiplied by transparency. Cleaner compositing, no color fringing at edges.
In compositing, you constantly work with two worlds: one where alpha channels are permanently married to RGB values (premultiplied), and another where they exist completely independently. The unpremultiplied image — in English technical jargon unpremultiplied — is the second variant, and it's your tool when you want to deliver clean work.
The practical aspect: In an unpremultiplied image, the RGB color channels are not multiplied by the alpha channel. This means your red remains red — regardless of whether the alpha value at that point is 1.0 or 0.1. Unlike with a premultiplied image, where a partially transparent red area is already pre-calculated to dark red, here you store the full color information separately. This allows you three crucial things: Firstly, you can change the alpha later without the color fading or appearing dirty. Secondly, you avoid those characteristic dark or bright halos at edges — which often occur when premultiplied images are overlaid on a background. Thirdly, you can apply color adjustments (color correction, grading) without the transparency information suffering.
On set or during rendering, this doesn't matter much — you take what comes out. But as soon as you're in the compositing system, be it Nuke or After Effects, you make a conscious choice: If you import your EXR sequence with alpha, ensure the software interprets it as unpremultiplied — or convert it in the first step. With unpremultiplied images, your keying runs cleaner, your rotoscope work becomes less fleeting, and when you composite in layers, noise or color casts don't suddenly add up.
Practically: For VFX shots that go through green screen or other keying processes, or for roto-intensive sequences, you should always work in the unpremultiplied space — this saves you touch-ups at the edges in the end and significantly increases output quality. Most modern rendering engines support this without problems; it's more a matter of workflow discipline.