Relevance sorting is the "default" sort order returned in DISCO when any search is run in the search bar. Relevance sorts results according to their degree of responsiveness to a search. It's most commonly applicable to words or phrase searches, but applies to all DISCO searches. In this way, DISCO tries to give you the results that you want, and in the order that is most important. Web search engines, such as Google, perform something similar when you run a search.
In general, relevance sorting is based on an algorithm that creates a score created using the number of "hits" for any search term, and weighted according to the relative location of the hits. For example, a document with five hits is more relevant than a document with a single hit. Additionally, a hit in a subject field is accorded slightly more weight than a hit in the body. If the result of the algorithm is a "tie" then DISCO will sort the ties according to ID numbers.