External transactions refer to transactions received via the p2p network as opposed to transactions created and saved via the send RPCs.
The existence of silent payments transactions creates a new challenge for RBF. The addition of a new input to the input set of the original transaction, requires regeneration of the original transaction's silent payments outputs for the replacement transaction. This process requires the original SP payment instructions; #35302 persists these payment instructions for this purpose. However, if the transaction being replaced is an "external" transaction, the payment instructions are not available to the wallet.
There is no way to identify silent payments transactions, so a possile solution is to flag transactions that are external and require that the user explicitly force the replacement of such transactions. A new parameter should be added to the bumpfee RPC that must be set to true by the user to allow the replacement of an external transaction.
In addition to this, a new parameter should be added to the bumpfee RPC to allow SP payment instructions to be provided.