There is an option to "only show when...." however there is jack in the way of information on how to just say "show never" or "only show when visible" (i.e. link a breadcrumb trail)
In this case I want to call the interface directly from its URL based on other conditions (I have 20-30 such pages which I don't want crowding out the top navigation)
I only want the top navigation menu to show maybe 2-3 pages, not 50. But it seems every interface gets added to the top navigation whether I want it there or not.
How do I set this up?
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If you want a URL you can call externally, you either have to add it as a site page or you'll need it to be a record view.
Alternatively, you can create your top level pages and have a secondary navigation which shows or hides things with dynamic links but the sub pages will not be accessible via a URL.
You can find examples of secondary navigation on Appian's design page: