I searched the convention tag, but the top google search result says, "the convention tag has no usage guidance."
I have no idea which sense does this tag relate to. When should I use this tag?
"The X tag has no usage guidance" means that the tag has no tag wiki/excerpt. You can see the full wiki by clicking on a tag, then clicking on "Learn more...", which in your case should take you to this page.
It looks to me like this tag is used on questions that have next to nothing in common. "The way things are done" isn't a particularly descriptive category. In other words, it doesn't look like a good tag, so my advice is to not use it. In fact, it could probably be burninated, if enough people care to edit.