What do the "Other" and "Developer" options in Spotlight index?
This is sort of a half answer, half response containing observations and guesses in response to bmike's comment.

It looks like the "Developer" option is for files that macOS considers to be development related files.  For example, I searched for .h, .html and .m files and they were listed under the "Development" category:



"Other" must be for anything that macOS can't make a good guess at.  For example, I searched for .json files and actual .json files showed up under "Developer".  However, files that had unknown or missing extensions showed up under "Other":



I'm still not sure of the complete criteria that macOS uses to discern what files fall into what category.  Additionally, it looks like Spotlight won't even try to display a preview of files in the "Other" category even if they are textual.