What Is the Difference Between RBF and RBF?

As far as I know Bitcoin had some sort of replacement built in from the Satoshi's days from the get-go or you know from the time he was working on it. However as far as Iknow it was disabled citing well I'm not even sure if he was citing this but the explanation was that it was to protect against denial of service attacks. Then at some point there were various discussions about re-enabling RBF in one form or another whether you know the mempol policy level or well at some point at the transaction level and that's really what what happened or optin RBF is what we have today which is a flag that you can set on transactions.