Relevance:

By default, results are displayed according to the relevance ranking of the search terms in the metadata.  Moreover, Relevance is based on the occurrence of the search term in shorter fields than in the bigger. Example: If user searches for the term ‘moisture’, metadata containing moisture in the ‘Title’ field will be displayed on top than the metadata containing ‘moisture’ in the ‘Abstract’

In more technical terms:

Scoring uses a combination of the Vector Space Model (VSM) of Information Retrieval and the Boolean model to determine how relevant a given Document is to a User's query. It uses the Boolean model to first narrow down the documents that need to be scored based on the use of boolean logic in the Query specification.