http://en.wikipedia.org/wiki/Continuing_patent_application

see excerpt below, only IMO

Continuation-in-part

A "continuation-in-part" application ("CIP" or "CIP application"), claiming priority based on the filing date of the parent application, is one in which the applicant adds subject matter not disclosed in the parent, but repeats substantial portion of the parent's specification, and shares at least one inventor with the parent application. The CIP application is a convenient way to claim enhancements developed after the parent application was filed. It is the successor to the earlier "additional improvement" patents mentioned above.