Another thing I found out thanks to redonihunter further tests. Searching for "fishing" will display a header on the search page suggesting and linking people towards Browsing the tag for "fishing". It will not show up for boxing, because boxing seems to be considered a "low power tag".
They are sometimes called featured or suggested tags. I think suggested tags might be the "offical" name.
You will find non suggested tags with quite a lot of items. Uhem, nsfw has like 5000 items. The adult tag is better though, to use as a tag, since it is suggested and you can only have 10+1 tags.