title: What's the meaning of user property of maven plugin parameter
body: <p>I am a newbie to Maven.
  When I try to refer to any maven plugin document, I always see following format for the parameter definition:
  Name                     Description
  {parameter name}         {description}
                           Default Value is: ...
                           <strong>User property</strong> is: ...</p>
  
  <p>For most cases, I saw the user property is the same with parameter name.
  I wonder here what's the difference between "User Property" and "Parameter Name". I think the parameter name should be element tag name when specify the parameter value of the correspond plugin when configure them, but what's the usage of "User Property"?</p>