I have noticed that the Finder's **Recent** Smart Search Folder doesn't show several type of 'app document' file.

The same is true if I create a new Smart Folder with the criterion set to `Kind = Document`.
(You can sort through the results by Kind in a List View)

For instance: Affinity Suite documents, such as `.afpub`, and `.afdesign` are **not** included.
Omnigraffle and OmniOutliner files **are** included. 
Files from other apps, such as Finale (`.musx`) and Dorico (`.dorico`) are not included. 

If I change the search criteria to `Kind = Other: Affinity`, then I get the Affinity documents, so it's not like Spotlight hasn't indexed them.

Does anyone know on what basis Spotlight considers a file to be a document?