Functions, known internally as
ValueSources, are typically mathematical in nature; they take constants and references to single-valued fields and other functions as input to compute an output number. Functions complement typical queries by enabling you to boost by a function, to sort by a function, to return a value from a function in search results, to filter by a range of values from a function, and they can be used in clever ways wherever Solr accepts a query, such as
facet.query. They are very versatile, though they are usually only used for custom relevancy boosting.
A function is often referred to as a function query although that is a little confusing, as it makes no distinction with actual function queries ...