@links and related categories (relcats) are both methods of linking together two categories.
They are extremely important for user navigation between categories on related topics.
The type of link used depends on the relationship between the categories.
When to Use @Links
@links are used to link from one category to another that could theoretically
be a subcategory of the first.
A relcat is used because surgery is a medical specialty.
However, medical specialties are not a subset of surgery; the relationship
is the other way around. I.e. medical specialties could not logically
be a sub-category of surgery, but they are related, thus a relcat
link is used.
Indeed, as a general rule, if an @link is used from categories
A to B, a relcat should be used from B to A.
Another use for relcats is to link to categories that are
related and in different languages but not equivalent. To link
together categories that are the same but in different languages,
you use an altlang (alternative language link). If, however,
the relationship is not 1:1, a related category link should be used.
