If a file in the partial application does not exist in the deployed application EAR, then it is added to the deployed EAR file as a result of update operation.
If there's a "nice to have" package related to yours, but not required for normal operation of your package, then you should use the "Recommends" or "Suggests" fields instead.
If you try to use a selector where the operation name does not match, then you get a ServiceRuntimeException saying the operation name cannot be resolved.