There's 10 questions with that. It lacks a tag wiki. Is it a useful tag?
1. Does it describe the contents of the questions to which it is applied? and is it unambiguous?
Sometimes it's being used when asking whether to use "that" or "who" What is the mistake in "There are many Danes who speak English"?
Sometimes it's being used to ask whether "that" can be simply dropped: Is it required to use "that" here?
2. Is the concept described even on-topic for the site?
3. Does the tag add any meaningful information to the post?
I guess it's useful to have a tag, because you can't google for the word "that".
4. Does it mean the same thing in all common contexts?
Pretty much the same as number 1.