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