What exactly is the purpose of the scene's draw mode Alpha Channel

The manual say’s this:
Alpha Channel: render colors with alpha

for UI elements if I set a value between 0 and 255 for the opacity, they’re shown in transparent black. Otherwise they’re white.

This doesn’t work with the transparent objects using the standard shader.

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It seems to render similar to the cutout function of a shader.
When a pixels alpha is below a certain threshold, it doesn’t get rendered.
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The alpha channel mode seems to render alpha=100% white and alpha=0% black. And in betweeen like you see in the picture with the text. The border of the text has blured alpha therefore it is grayish.