Terms in an equivalence relationship refer to the same concept (or sometimes a roughly equivalent concept). They are usually synonyms. As has already been discussed, there should be only one “indexing term” or preferred term in your thesaurus for any given concept. You can and should include synonyms of those preferred terms in your thesaurus as synonyms. And the way to do that is with equivalence relationships. The synonyms that aren’t “preferred terms” are “non-preferred terms” in the same term record. Your taxonomy software instructions will indicate how to add non-preferred terms to term records; this action will establish an equivalence relationship between the preferred term and each non-preferred term.