Tag implications are used when all images belonging to one tag should also be tagged with another, broader tag. For example, 4koma implies comic, and fate/stay_night implies fate_(series). Since implications are applied automatically, they are not used in situations where one tag is only commonly, but not always, tagged with another.
For the most part, implications are only used when both tags are of the same type (general, copyright, character, or artist). Character tags do not imply their respective copyrights, as the maintenance burden for such a system would be too high and of questionable benefit (see topic #10624).
When two tags refer to exactly the same concept, an alias should be created instead.
You can request that an implication be created using the bulk update request form.