MASKING AND COVERAGE: AGAIN

I am at the point now where I have zero confusion except this masking
business vs. grouping. Any further comments would be appreciated on this.

*MY ORIGINAL COMMENT*
For modes like DstIn masking would thus have no effect.

The blend function for RGBA excepts a value from 0 to 255.
I also have a global alpha value which affects all alpha.

My theory is that no matter the blend mode or combine mode that source
pixels can simply have the alpha adjusted by multiplying with both
global alpha and the mask value, which can optimally be combined prior
to actually calling into the blenders.

>>No, if you have grouping, you can't simply redistribute alpha. That will
make the graphics interact with each other which is usually not desired.

What does it mean "can't simply redistribute the alpha"?

If I take a masking value of 255 and simply multiply it with the source
color alpha. The source color does not change. But for the edges of the
polygon where it is antialiased the mask value might be 64 for example, so
multiplying that with the alpha of the color and then calling into the
blender/composite function is surely the way to go?

Any notes on how the grouping affects this simple alpha masking will help.