Continuation in parts - patents

Hi,

As much as i know, continuation-in-parts (CIP) for patent laws is that you can add claims and things to your previously filed and approved patents if you found improvements to your invention later, is that right?

However, i am confused about how some websites say "CIP must be filed only for PENDING patents" while USPTO websites and some other websites simply said "CIP can be filed during the parent's patent LIFETIME".

Please advice

Thanks