PSD file with 16 bit mode

I read a psd file format specification document which is downloaded from
internet. As far as I can see,  the last part of the file is image data
section, which is precombined image.
>From the document

Photoshop CS File Formats
Specification<http://libpsd.graphest.com/files/Photoshop%20File%20Formats.pdf>
page
47

it saids that The image data. Planar order = RRR GGG BBB, etc.


The problem is when I try one of my psd file with 16 bit, rgb mode with
alpha, the order of saved channel is RGBA
And 8 bit mode,  the order is ARGB.


I could not find any clue why it works this way, wonder is there any place
in the psd file decide the order of these channels.

Or is there any official psd format specification from adobe, I could not
find it.


Thank you in advance.

Chiyu

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