So this means that when a transaction is broadcasted it has a flag saying that RBF can be enabled, or that it can't?
If all of the inputs of a transaction have nSequence = UINT_MAX or UINT_MAX-1, then they have RBF disabled. Sending transactions with nSequence = UINT_MAX is the default for ~all wallets.
Does this mean that transactions that need to change nSequence so they can use CHECKLOCKTIMEVERIFY will be forced to enable RBF?