GLOSSARY
There are two methods of storing alpha in a bitmap: premultiplied and non-premultiplied.
To composite an image that is in non-premultiplied format, the alpha must be multiplied by each of the R, G, and B channels before adding it to the color of the background image. This provides the correct transparency effect, but it must be done each time you composite. With pre-multiplied alpha, you store the R,G, and B components with the alpha already multiplied in, so compositing is more efficient.
This is not the only reason that 3DS MAX stores images in the premultiplied format. When you render an image, you typically want the edges of the objects to be antialiased. This effect is achieved by determining the fractional coverage of pixels on the edge of the object, and then adjusting the alpha of the pixel to reflect this. For example, a pixel that is 30% covered by the object will have an alpha of .30. To antialias the edges, the alpha must be pre-multiplied to darken these edge pixels. (This is basically equivalent to compositing the image over a black image). So it is natural, in a sense, for rendered images to have pre-multiplied alpha. If you didn’t pre-multiply in the alpha of a rendered image, then just looking at the RGB you would see jaggies on the edges of objects -- you’d have to composite it against black using the alpha channel whenever you wanted to display it.