A continuation-in-part patent application can be thought of as a part-child of a prior filed “parent” patent application. A continuation-in-part application is sometimes referred to as a CIP application.
A CIP application contains some or all of the subject matter from the prior parent application and some new matter that was not in the prior parent application.
The Manual of Patent Examining Procedure provides that a continuation-in-part is “an application filed during the lifetime of an earlier nonprovisional application, repeating some substantial portion or all of the earlier nonprovisional application and adding matter not disclosed in the said earlier nonprovisional application.” MPEP 201.08.
A CIP application can have claims with different priority dates depending on whether a given claim is completely supported in the parent application or whether it contains new matter that was not supported in the parent application.
Unless you need a split set of claims where some claims are completely supported in the original parent application and some are not, then you might want to consider not filing the application as a CIP but rather as just a new application. This is true because the downside of a CIP is that the resulting patent life (term) will be measured from the filing date of the parent application. Therefore, if you do not need the benefit of the parent application, then you will probably want to longer patent term afforded by not claiming priority to the parent.