The post in this thread that I am seeing with the number 5 reads
Although the sequence of filters is edited in the Dynamic Content Filter dialog, it appears the merged projects filters order cannot be arranged. Can anyone confirm?
Post 12 looked like you were quoting something.
I'll set up a merged responsive output with filters to see what does and doesn't work.
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@petergrainge
I set up a parent and three child projects all with the tags A, B and C.
I created three paragraphs and applied one tag to each.
In their outputs, I defined the filters as A B C in that order.
It worked without issue.
Then I reordered the filters to C B A in each project and generated all again.
The filter order showed as C B A and still worked.
Finally I turned off filtering in all the projects except one.
The filter options still appeared as I would expect but they have no effect on the topics except in the project where I had left it turned on. Working as expected
Looking at your specific questions:-
Editing in the Conditional Build Tags pod does not automatically update the tags in the SSL properties Dynamic Content Filter dialog. This potentially can cause a filter tag mismatch.
Yes. Apply the DFA rule. Don't fiddle about (or something like that).
If Use Dynamic Filter… is not selected in child projects SSL Properties , child content will not filter, and will display with content from other child projects. Is there a way to display a filter for only one child without creating filters for all child projects?
No. The filters must be the same in all projects and will always display. However, they will only work on projects the the option to use filtering has been selected.
Although the sequence of filters is edited in the Dynamic Content Filter dialog, it appears the merged projects filters order cannot be arranged. Can anyone confirm?
Wrong. They can be in any order you like but that must be consistent across all projects. When you set up filtering, there are arrows to move the tags up or down.
Adding a Dynamic Content Filter list in the parent of a merge seems to prevent search links from opening? Can anyone confirm?
They work in the demo I have set up.
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I'm thinking the issue is with your tagging.
Contact me via my site with a link to this thread if you would like a copy of the setup.Include a link to this thread.
See www.grainge.org for free RoboHelp and Authoring information.

@petergrainge