In the API, data replication only reflects changes made to object records. It does not determine whether changes have been made to the structure of objects (for example, fields added to—or removed from—a custom object). It is the responsibility of the client application to check whether the structure of a given object has changed since the last update. Before replicating data, client applications can call describeSObjects() on the object, and then compare the data returned in the DescribeSObjectResult with the data returned and saved from previous describeSObjects() invocations.