The tag usage is pretty clear:
Questions that concern the use of expressions, parts of speech, words, verbs, etc.
The tag difference instead is, to me, absolutely obscure:
For differentiating between words, characters, pronunciation, syntax, etc.
I'm not sure how to suggest an edit to this tag wiki either, as I don't fully understand the current description.
Anyway, as of today, the tag difference has a total of 517 questions, which mainly revolve around, unsurprisingly, differences.
A few examples, from the top search results in this tag:
- Difference in word choice To make friends: 交朋友 vs 认识朋友?, also tagged usage
- Difference in usage What is the difference between 这个 and 此? also tagged word-choice
- Difference in usage The difference and usage of 刚才 and 刚刚 also tagged meaning
- Difference in usage Is it possible to know what 都 means in “Subj + 都 + Predicate + 了”? also tagged grammar and a bunch of other tags
- Difference in usage Difference between 趁着 and 就 also tagged usage
- Meaning nuances Difference between 搂抱, 喂抱, 拥抱, 抱抱? also tagged word
This is a small sample, but it seems to me that this tag is used in questions that are actually concerned about nuances or word usage.
The tag difference itself is bit weak. Potentially, almost anything that involves a comparison can be tagged difference so I'm concerned that it fails at categorizing content on the site in a meaningful way.
Counter argument: I can see how someone might use the tag along with some other keywords in the search bar. From one of the examples above, I might search for
[difference] 趁着 and fish up questions that discuss the difference between 趁着 and something else. (What difference that would be is another story).
In conclusion: I think that this tag adds noise, and could be synonymized in favor of usage. I already proposed the synonym, though I'm not sure I did it right. My proposal is to keep usage as the primary tag and have difference as a synonym.
Final disclaimer: I don't feel strongly about the tag synonym, but I do feel strongly about the current difference tag wiki, which is beyond my comprehension (clarifications are very welcome).