title: What is meant by mobility of entities?
body: <p>The book <a href="https://rads.stackoverflow.com/amzn/click/com/1430249269" rel="nofollow noreferrer" rel="nofollow noreferrer">Pro JPA 2</a> has to say this about <em>mobile entities</em>:</p>
  
  <blockquote>
    <p>Client/server and web applications and other distributed architectures
    are clearly the most popular types of applications in a connected
    world. To acknowledge this fact meant acknowledging that persistent
    entities must be mobile in the network. Objects must be able to be
    moved from one virtual machine to another and then back again, and
    must still be usable by the application.</p>
    
    <p>Objects that leave the persistence layer are called detached. A key
    feature of the persistence model is the ability to change detached
    entities and then reattach them upon their return to the virtual
    machine. The detachment model provides a way of reconciling the state
    of an entity being reattached, with the state that it was in before it
    became detached. This allows entity changes to be made offline, while
    still maintaining entity consistency in the face of concurrency.</p>
  </blockquote>
  
  <p>Can someone help me decode the information in the above two paragraphs? Why would some one need the entity to move between JVM's?</p>