[Koha] indexing of 450 authoritiy field?

Hello, I am currently working on my authorities and I was dissapointed to discover the information I have entered in the 450 MARC 21 authority field (see from tracing - topical term) is not retrieved in my searches. Is it not indexed? And if so, how can be indexed? Cheers, Vasiliki Βασιλική Μελά Βιβλιοθηκονόμος *Αρχείο της ΕΡΤ* vmela@ert.gr