This option is used to specify an Entrez query used to constrain the indexing queries. For example:

(((brewer OR baker OR budding) AND yeast) OR ((S OR Saccharomyces) AND cerevisiae)) AND (enzyme OR reaction OR substrate OR product) AND (kinetic OR parameter OR constant OR concentration OR rate)

This limits the number of documents indexed by keeping queries searching for individual concepts within a specific context. By doing so, the performance of the system is improved in both the run time and the quality of the results.