This applies only on CHM help. See Downloading GN4 help and Installing GN4 help on your hard disk.

Stemming feature in HTML help strips query terms of common suffixes that indicate, for example, plurality and verb tense. After query terms undergo stemming, viewer performs a search for variants of the resulting root words. Unless you turn it off, viewer performs query-time stemming by default, to accommodate the variety and imprecision of human languages.

In English, for example, if walk is your query term, stemming assures that variants like walks, walking, and walked are also included in the search; the query will match documents that viewer would otherwise overlook under a strict interpretation of the query.