Additional SFF Support
A Small Form Factor (SFF) transceiver is an optical transceiver that is soldered onto the PCB. It tends to be smaller in size than traditional SFPs.
There was a need for the ESR6300 to support SFFs in order to meet high vibration and shock requirements that can't be met with traditional SFPs. Therefore, support was added in 17.6.1 for the Finisar FTE8501K1LTN.
Since, this is not a Cisco transceiver, the end-user will need to use the service unsupported-transceiver CLI in order to get the router to accept and initialize the SFF.
Functionally, an SFF operates in the same way as an SFP.