I’m not sure about Site Search Category is meant for. I just found a forum post mentioning a wrong usage.
There’s no examples in the demo site. There’s no mention inside web documentation, except for Tracker API implementation.
Did anyone ever used it? 
Hi @tassoman
Yes, I already used it… But in most cases, just keyword is needed.
For example, in a site like google, there is just one field, then no category.
In some sites, you can search some words, but associated with a category. Example, if you search for tomato, is it as botany or as gastronomy? In most cases, the category should be provided in a listbox to the user…
Unfortunately, Matomo UI does not provide by a report that display keywords by category default. I did not try with custom reports, but it should work (I hope)
Hello @heurteph-ei thanks for your reply.
In other words, having a search category is more useful for an ecommerce website, for example.
Let’s think about keywords big, yellow, brandname, if those keywords are contextualized in a product category, let’s say for example shoes, shorts, women apparel, you can end up by having “green” searched inside “shoes” category.
Hi @tassoman
Yes, that’s the idea, but by default Matomo doesn’t display the keyword as sub level of category.
I just tested in custom report to try to get both keyword and category, but it seems it doesn’t work as well as I expected… 
Maybe create a ticket to Matomo for this?
As far I can remember, the Site Search report it’s one-level list.
So, the maximum level of achievement, can be different rows, having same keyword but different category: