Explanation of a the transparency of an .EXR file Simply put, what does the transparency data of an EXR file represent?
In an EXR, the convention is to store associated alpha, also known as premultiplied. The latter term however doesn't do justice to the nuances of the alpha format as compared to its evil distant cousin known as unassociated alpha, aka straight or key alpha.

In an associated alpha image, two facets are represented via the RGB triplet and its associated alpha channel:


Occlusion.
Emission.


In an associated alpha image, the alpha channel could be considered to represent a distillation of the geometry of occlusion covering a given pixel coordinate. 0.5 would represent a baked down 50% occlusion, while the RGB values would represent the degree of emission.

This also means that an RGBA triplet such as 0.0,12.4,10282.9,0.0 is entirely valid, and represents a pixel that is emitting and unoccluding. Naturally occuring things such as candles represent such a combination. Consider the following image:


The above image could not be created using unassociated alpha, and is symptomatic as to why ray tracing renderers cannot generate any alpha model other than associated alpha.

The fundamental difference between the two alpha formats is defined by the two different over operations, as described by Porter Duff.

Associated alpha over:

FG.RGB + ((1.0-FG.Alpha) * BG.RGB


Unassociated alpha over:

(FG.Alpha * FG.RGB) + ((1.0-FG.Alpha) * BG.RGB


The nuances of the difference mean that care should be taken when dealing with alpha and the RGB channels.