In this site, whenever I am looking for an optional tag to better fit my question into a subdomain, I can hardly find one specific enough.
For example, I cannot find promotion or conversion, and for types, there are only types and parametric-types. However, they are very important concepts in Julia. I cannot believe that there are so little we can describe. I think when I was using StackOverflow, I could always enough tags to attract people’s attention. In addition, with more optional tags, I could search more easily.
Besides, for categories, there are no many either. For example, in the Usage category, there are only 2: First step and Performance. Aren’t there more categories? And I also do not believe that there are only 15 sub-categories in the Domain category.
About the optional tags, playing with the post tool, you can create a new one, that can help. about the categories, it’s another matter, but i agree with you: there are not enough categories. the question to ask now is, what categories can be added?
In particular, i would like to see the “chemistry” sub-category 
Thanks for your reply. It appears that I cannot add one new tag. I do not know whether it is my browser problem. I click add, then nothing happens, the dropdown menu just disappears. I am using the latest Google Chrome on the latest macOS.
That’s a good idea. I was wondering why there is only one directly sub-category of Physics: Astro/Space. It does have a Modeling sub-category, though. But Physics should include more.
We initially created too many sub-categories in the domain section, some of them see ample usage, some of them are occasionally used and some of them are quite. A sub-category does take up space in navigation and on the main site.
Right now the semi-official policy is that we encourage the usage of tags, if there is a tag that is frequently used then it is warranted to create a sub-category that coalesces the posts.
Tags can be created right now at TL3 (Trust-Level 3), and above.
As an example @longemen3000 is at TL3 and that is why he can create a tag.
I hope this helps!
-V