I wrote some tag-wiki entries today and a thought came up that I'm unsure on how to handle. Some tags have a clear meaning and its usage is mostly clear (linux goes to linux related questions). Other tags should have a notation on when to use them, and when not. The Tag Wiki help says:
The full tag wiki is a detailed introduction to the topic, suitable as a destination for those curious about it: ► what questions should have this tag?
So the part of "Don't use bootable when you are looking for a OS because being bootable is an intrinsic element of it." would go to the full tag wiki.
A good example for this situation is the database tag (as has been described well in @Izzys answer).
But I think that only few people will read them because they rush here, ask a question and only read the pop up that tells them what the tag is about (if anything at all).
That would mean that we should stuff such info into the excerpt.
Example (bootable, currently not yet reviewed)
- Indicates software that can boot a computer on its own.
- Use this if you are looking for programs that come on or can be put on a bootable medium (cd, dvd, usb-stick...) and then brings up the machine.