This attribute characteristic applies only to three-tier Java Client or Cocoa client applications. It plays a vital role in the process of business logic partitioning. This is the process in which you choose the data that is made available to the client application.
For example, a Customer entity might include an attribute
called creditCardNumber.
While this attribute is probably important to the server-side application
for processing orders, it is considered sensitive data and should
not be made available to client applications. To ensure that client
applications don’t have access to this attribute, it should not
be marked as a client-side class property.
See Inside WebObjects: Java Client Desktop Applications for detailed information on the client-side class property characteristic, including a tutorial.