Ahh ok! that makes sense. So they are just confirmations that the ack’s to received packets have been sent by the RFM. The message text could do with some editing in that case. The current log message is derived from seeing the “send” confirmations being in the format of “4b” to tell us a frame of 4 bytes was sent so “confirmed sent packet size: → 4b” made sense, apparently less so for an ack confirmation.
[this is another note to self, i can revise the messages if I know the full range of utilization]