See here: http://sharepoint.stackexchange.com/questions/21349/purpose-of-the-mappings-caml-element-in-a-custom-choice-field-definition.
Short version: there is a "feature" in CAML that is referenced in the docs, but only appears in old version of the CAML schema definition documents (documents available only from an obscure page found in the basement of a remote office, near a warning notice for demoliton of an house and a sign "beware the jaguar"). Not only the feature doesn't seem to work, but nobody knows what it was intended to do in the first place. As a cherry on the cupcake, the meta…