1. A computer-implemented method for at least one of updating and using a data store of words associated with subject matter categories, comprising: comparing, by a computer processor, words of a received document to words of a data store that are tagged with respective indications of subject matter categories to which the words of the data store belong; for each word of the received document that does not match any of the tagged words of the data store, determining, in a first determining step and by the processor, whether a frequency of the word in the received document meets a frequency threshold; and for each word for which it is determined in the first determining step that the frequency of the word meets the frequency threshold: tagging the word, by the processor, as belonging to a subject matter category with which the received document is associated; and updating the data store, by the processor, to include the word with its tag.