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.
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.
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.