You will have to make up your mind whether you wish to create a "tagged PDF" or an "ordered PDF".
Having a PDF that is both tagged and ordered can be challenging if not impossible in a number of cases. The international standard for accessible PDF, PDF/UA (ISO 14289-1), makes it very clear that only tagging is relevant for guaranteeing universal accessibility, and 'ordering' of page content items is not relevant.
Ordering used to be relevant in the past as a workaround when using tools that nowadays should only be of historic interest (such as the reflow mode in Acrobat). Relying on tagging structure (as opposed to following the sequence of page content items on PDF pages) allows for a much richer and more predictable user experience.
Thanks Olaf, your answer confirmed what I believed to know about the (less) sense of the order panel. Unfortunately, it is not that obvious if you are not familiar with the ISO definitions and just using the Acrobat interface. It makes it even harder to understand if you have read tutorials which explain to use the order tool to fix the reading order.
If you are open the touch up reading tool in the latest Acrobat version, the content is shown in the order view (with the numbers on the top left corner). I guess the touch up tool stays as the accessibility tool and wont be removed in the future. Why does it use this kind of "old" information. It's confusing.
I'm a bit surprised that the reflow mode should be historic. I think this view makes totally sense and is comparable to responsive web design. I think it would be better if the reflow mode would use the tagging order. Anyway, somehow I believe there are not a lot of people use this mode.
I hope Adobe makes it more clear in future Acrobat releases what tools really matters.
Actually, if you aer able to find the 'i' icon in the upper left of the Order panel (in my case it is black on dark gray...) and click on it, you will see an alert stating:
The Reading Order panel provides a visual representation of the order in which content is read by Adobe Acrobat and Adobe Acrobat Reader's Read Out Loud text-to-speech tool.
To modify the order in which content is read by assistive technologies, please use the Tags panel which displays the document's logical structure order.
So guidance does exist - it is just a bit hidden...
And regarding the concpet of reflow: I absolutely agree that the concept is ultra-important, but the implementation in Adobe Acrobat needs to move away from the current implementation and instead offer a reflow based on the tagging structure (and while at it, they may also want to do something about things like white content objects on white background, and similar limitations of the current implementation...)
Thank you for this information. I'm looking forward for the new implementations.