Placed INDD files are treated like graphics. Use the book feature instead to create the PDF.
I don't understand. The book feature outputs separate Indesign files as separate pages. In this case, I have placed a InDesign layout in an area of a page of another document with other graphic elements. The native InDesign document appears to be treating it as regular graphic. I can tag it as a figure and include it in an article, but when I open in Acrobat the placed ID file appears as a document with real text and graphics, but no tags and no reading order. To solve the issue, I had to create a PDF of the InDesign file, then place the PDF into the other ID file. I just thought I would be able to tag a placed ID file and it would work. Apparently I am the first person to try this, simply doesn't work. Adobe hasn't updated their accessibility tools in years. There has been a long standing bug where reading order doesn't follow the article order even if you check the use for reading order in the panel pull down (top down) but follows the layer order (bottom up) - drives me nuts.
You can't do it that way. Placing an INDD files creates internal a PDF with reach content and is used like any other PDF IMAGE. Tagging cannot be passed through.
You have to place the content directly, not via PDF or INDD files which are place. You can also use the place content tools to create linked content, but not placing an INDD in INDD for that purpose.