@links and Related Categories
@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.
Support Groups@ is a pointer to Health: Support Groups: Surgery. An @link is used because Health: Support Groups: Surgery could logically be a sub-category of Health: Medicine: Surgery. i.e. surgery support groups are a subset of surgery.
When to Use Related Category Links
Health/Medicine/Surgery also contains a related category (relcat) link to Health: Medicine: Medical Specialties.
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.