Sometimes, two tags can mean EXACTLY the same thing. For example, pantsu and panties have identical meanings. It makes sense that if you search for one, you should also get the results for the other.
In other cases, a tag may be mistakenly used so often (for example, a very common typo) that it needs be automatically redirected, an existing tag may need to renamed in perpetuity (for example, if an artist changes their name), or a tag may impractical to maintain as distinct despite being notionally different (for example, the difference between magenta and pink hair).
This issue is addressed by using tag aliases. You can alias one or more tags to one reference tag.
For example, if we aliased pantsu to panties, then whenever someone searched for pantsu or tagged a post with pantsu, it would be internally replaced with panties.
Remember that the tag aliased to another will be emptied and redirect to the tag. If you are not sure if the two tag are totally equivalent, implications may be what you want.
The reference tag cannot be aliased into any other tag, but can be used in implications.
Use caution when adding aliases as it can be a destructive process. When a tag is aliased to another tag, that means that the two tags are equivalent. You would not generally alias rectangle to square, for example, because while all squares are rectangles, not all rectangles are squares. To model this sort of relationship, you would need to use implications.
It should be noted that the tag system is now bilingual (example: maburaho should get the same results as まぶらほ when used as a search) so kanji, hiragana, and katakana should only be used for the Japanese tags.
Don't forget that aliases should only be used with consensus, apis_mellifera for example should not be the main tag for bee, if only a few tags use the "Apis mellifera" tag and hundreds use the "bee" tag.
In that particular case the "Apis mellifera" tag should not be aliased to the "bee" tag either, since all bees are not "Apis mellifera" ("Apis graeca" for example).
Implications could be a good option.
Aliases are very relevant in case of a character with many names, but choose wisely the new "reference tag". (Don't forget, for example, the English/Japanese order for character naming.)
While anyone can suggest an alias, only an administrator can approve it.
If you think you've found one or more aliases that are wrong, you can report so in this topic: http://chan.sankakucomplex.com/forum/show/308.