In the new, real-time segment builder, you cannot build segments and use contacts from a "customer" field as the resipiant. This is a great limitation in the new segment builder, and limits the adoptation of the new segment builder, since this is possible in the old, outbound segment builder. Use of "customer" fields is supported in other parts of real-time marketing, i.e. Dataverse triggers. "Customer" fields is a standard feature on standard tables such as case, oppotunity etc., and the ability to use "customer" fields would enhance the segment builder in real-time marketing.