First some general meta 
On my forum I used decent amount of categories. Mostly because of I had same structure on another sites and everybody is doing same. But as everyone knows, or should know, a lot of categories and plenty of sub-categories plus all sub-sub-categories are almost impossible to handle. And the most important part: categories donāt help an user to find relevant content.
Nothing new actually. That is one reason why Discourse supports only two level categories and using tags are guided so often.
Quite many are against tag, though. I reckon it leads to wide misuse of tags on another platforms, mixing them with keywords (partly tags and keywords are same) and hashtag abuse by Instagram etc. Plusā¦ common point on view is because of that history ordinary users donāt know how to use tags.
They can. For them, what they see on the screen, tags are same thing than categories ā but gives better results (IF used right by admins).
But tags work best with searches. And that is an issue too. As tags are widely misused so it is searches ā because it just doesnāt work. Users had learned not to use search, because it gives too many hits. It is neverending fight between skills of users, should all hits shown to an user or can we use someking guessing like Google is doing.
That is an issue devs must solve out. But it is different story.
Anyway. Mine audience are just ordinary people, not tech-savvy what so ever. They have difficulties to start use composer of Discourse and they couldnāt change theme. Sure, they can now, mostly, but it needed a quite lot guidance.
But they adapt lack of categories and started use tags right away. For them it was really intuitive act. Or as a couple users said: clicking tags is like searching, but better. All I needed to do on guidance level was just to tell then click tags and find out what happends and tell where Tags on hamburger leads.
Back to business
Discourse has one of the best search engines on free web-world. It is not perfect, of course, but is far superior than native search of wordPress or another CMSes (if you donāt know WordPress search plugin Relevantti, you should check it out; it is quite good).
Iām teaching now my users how to improve search results. Actually even that was much easier task than I expected ā and those users almost never use search anywhere (I donāt blame them, because searches sucks so often).
But now Iām lacking some references. I know Discourse can use plenty of tricks right in the search terms ā but those fragments and bits of knowledge are spreaded inā¦ many posts and topics.
I know ā writing document and tips&tricks need more work than coding itself. Iāve done documentation quite lot. It is not just to write one, but there must be at least three different documents depending of need and skills of users. Plus someone should keep those updated too.
Thatās why I totally understand I couldnāt find overwhelming guide of mastering searches on Discourse (or I justā¦ canāt search)
What Iām asking from community is
- simple listing for (or of; sometimes I hate english so much) search parameters or just examples/links
- teach your users to use search, most of them just canāt
- dump categories and start using tags instead (well, it needs some planning to do right, though)