Synonymous meta tags about tagging
Both exist, because they refer to different kinds of requests.
Tag synonym requests
These are about creation of tag synonyms, i.e. connecting two tags such that using them is equivalent, with both essentially having the same questions list. An example is osx-lion (the "real" tag) and osx-10.7 (the synonym).
While users can, in theory, set up tags themselves, this requires answer scores of 5 or more for users that'd vote about it, which is often too much, so this requires diamond moderators to handle.
These are mainly requests to disambiguate ambiguous tags (like e.g. cd, which can be both compact disks and the
cd command line command), or changing a tag's name, or maybe deleting useless tags.