You can enable the Complementary Series option by creating or editing a series under an existing series type. With this option, you can tie a Complementary Series with a zero dollar value to another series purchase or usage. So if the idea is to give a "buy one get one" or link two series together, you would assign the Complementary Series to be triggered upon purchase of your single class series. Another way this can be used is to assign the Complementary Series to be triggered once another series has been used (ex: 10 classes used out of a ten-class package might trigger a free class). This option allows you to report on this "giveaway" as a separate series.