afs:relevance can either include basic or rich information. By default, basic information is provided. Rich information is delivered when afs:explain=relevance is added to the query.
In basic mode, afs:relevance has one mandatory attribute rank. Its value contains the position of the document in the reply set (the document at position 1 is the most relevant document).
In rich mode, afs:relevance has three other attributes: pathlen, words, weight and fieldMatch: