The Metamorph engine works on the basis of set-logic. Each term in the query is checked for membership in each record, and if the appropriate terms are present the record is counted as a match. Terms can be required, excluded, or permuted. Required terms must be present, excluded terms can not be present, and there must be at least the desired number of intersections of the permuted terms. The number of intersections is one less than the number of terms present, so specifying @0 requires at least 1 premuted term be present.

Each term consists of a set of things that will match the term. A term can be a single word, a set of words, the set of strings that match a regular expression, a numeric range, or approximately match a term.