Invoked before indexing a IndexableField instance if terms have already been added to that field. this allows custom analyzers to place an automatic position increment gap between IndexbleField instances using the same field name. The default value position increment gap is 0. With a 0 position increment gap and the typical default token position increment of 1, all terms in a field, including across IndexableField instances, are in successive positions, allowing exact PhraseQuery matches, for instance, across IndexableField instance boundaries.
