Each index contains information of a global (i.e., index-wide) nature about each document and each feature. This information is called the document's or feature's score (in IoAddrIntern or DeIntern, respectively) and is used in the retrieval process to assess the relevance of a document, its retrieval status value (RSV) with respect to a query.
Index.update() invokes IdxIntern.update() which invokes e.g., IdfIdxIntern.updateSources() and IdfIdxIntern.updateFeatures(). The updateSources() method sets the score for each document in the index. The document score is a function of the frequency of each different feature extracted from the document. The updateFeatures() method sets the score for each feature. The feature score is a function of how many documents contain the feature compared to the total number of documents in the index. For IdfIdxIntern, updating of source and feature scores is multi-threaded.