Of course it works, it's the order. 
For me TeX wouldn't be the same without you. Thank you very much Hans.

Hans Hagen via ntg-context <ntg-context@ntg.nl> schrieb am Mo., 25. Juli 2022, 09:20:
On 7/24/2022 11:35 PM, cryo shock via ntg-context wrote:
> Hey there,
>
> I am having trouble understanding some basic blank page behavior. I hope
> that somebody of you can point me in the right direction.
>
> My primary goal is to create application documents consisting of various
> .tex components and with external PDF pages inserted as well. The second
> part of that goal is to use multiple-level bookmarking as well.
> Bookmarking should look like this:
>
> - Letter
> - Resume
> - Testimonies
> -- Testimony A
> -- Testimony B
> -- Testimony C
>
> In the following MWE you can see my progress along with some notes to
> help understanding.
>
> There are two questions, I'd wish could be answered:
>
> 1. Why does TeX insert a blank page before a pagefigure I enclose within
> a (hidden) head?
>
> 2. Can the \bookmark[list]{text} command be somehow used for multi-level
> bookmarking?
>
> While the first question would make up for a more consistent solution,
> the second question would do the job as well.
>
> I hope some of you can enlighten me.
>
> Cheers, k
>
>
> %%% START of MWE
>
> \definehead[VisiSection][section]
> \setuphead[VisiSection][number=no]
>
> \definehead[InvisiSection][VisiSection]
> \setuphead[InvisiSection][placehead=no]
>
> \definelist[CustomBMarks]
>
> \setupinteraction[state=start]
> \placebookmarks[VisiSection,InvisiSection,CustomBMarks]
>
> \startcomponent * % not a product, no
>    \startVisiSection[title={Custom Section A}]
>      Text of Custom Section A
>    \stopVisiSection
>    \page
>    \startVisiSection[title={Custom Section B}]
>      Text of Custom Section B
>    \stopVisiSection
>    \startInvisiSection[title={Hidden Custom Section A}]
>      Text of Hidden Custom Section A
>    \stopInvisiSection
>    \page
>    \startInvisiSection[title={Hidden Custom Section B}]
>      Text of Hidden Custom Section B
>    \stopInvisiSection
>    \page
>    TEST\bookmark[CustomBMarks]{Start of Teststack} \dorecurse{800}{TEST
> }NEEDLE\bookmark[CustomBMarks]{Needle in the Teststack}
> \dorecurse{400}{TEST }TEST.\bookmark[CustomBMarks]{End of Teststack}
>    %%%
>    %%% So far no problems.
>    %%% The following combination does the job, but I see no possibility
>    %%% to arrange the "CustomBMarks"-list in multi-levels.
>    %%%
>    \page
>    \startstandardmakeup
>      \filterpages[{test.pdf}][1]\bookmark[CustomBMarks]{Pagefigure}
>    \stopstandardmakeup
>    %%%
>    %%% So the following method would be the best choice to work with
>    %%% multi-level bookmarks. But as soon as I start working with
>    %%% (custom) heads together with pagefigures, CTX places a blank
>    %%% page before every figure...
>    %%% Note: you don't need a dummy file. The last (unnumbered) page is
>    %%% the wanted external PDF. The numbered page before is the unwanted
> one.
>    %%%
>    \startInvisiSection[title={External Testpage A}]
>      \startstandardmakeup
>        \filterpages[{test.pdf}][1]
>      \stopstandardmakeup
>    \stopInvisiSection
> \stopcomponent
because even a hidden head is something (anchor on page), you can try this:

     \startstandardmakeup
   \startInvisiSection[title={External Testpage A}]
       \filterpages[{test.pdf}][1]
   \stopInvisiSection
     \stopstandardmakeup

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