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With transparent objects the alpha normally takes into account the amount of transparency of the object and renders with varying degrees of grey - for example very dark grey for very transparent objects. It is possible to render a completely white alpha also for transparent objects with the Opaque option. This can be useful if you want to maintain the full look of the transparency from the render, instead of blending it with any other images in your composite. 
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From left to right: Render, Alpha with opaque option OFF, alpha with opaque option ON
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An example showing why this is necessary:
Comparing two renders - with/without a black background. Both renders would produce the same alpha map and the illumination on the sphere itself is exactly the same. The sphere has been rendered deliberately out of focus to show what happens at the edges of objects (which always fade into the background even when in perfect focus because of the antialiasing applied).
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The first sphere has a background which changes in brightness and the out of focus parts of the sphere mix with this changing background. After using the alpha to cut it out of the background, those areas with changing brightness remain. It is not possible to remove them using either "remove black matte" or "remove white matte".
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