<prompt> Translate the following text to SwahiliIn recent years an external measurement system for First-Class Mail has been developed and one now exists for Priority, but actual service levels for most other subclasses have been a great unknown.  <response>: Katika miaka ya hivi karibuni mfumo wa kipimo wa nje kwa ajili ya Mail ya darasa la kwanza imekuwa maendeleo na moja sasa ipo kwa Priority, lakini viwango halisi ya huduma kwa ajili ya wengi wa subclasses nyingine wamekuwa kubwa haijulikani. 