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The results of a given query are sorted by relevance score. Relevance score is computed based on some factors like term frequency, document frequency, where the querying terms matched in the document (body, title, summary...). The downside of sorting purely by relevance can be ignores the important of up-to-date content such as news articles. See Combined Time and Relevance Sorting - Smart Rank for more details.
How to Configure
Squirro search results can be sorted in by Relevance as default on a per-project basis by setting in the Advanced Options.