The full form of SFM is Selective Forwarding Middlebox

This is the “official” specification term given to the SFU (Selective Forwarding Unit).

Another method for handling media in the RTP mixer is to “project”, or make available, all potential RTP sources (SSRCs) into a per- endpoint, independent RTP session. The middlebox can select which of the potential sources that are currently actively transmitting media will be sent to each of the endpoints. This is similar to the Media- Switching Mixer but has some important differences in RTP details.

SFU stands for Selective Forwarding Unit. Also known in the specifications as SFM (Selective Forwarding Middlebox). At times, the term is used to describe a type of video routing device,

while at other times it will be used to indicate the support of routing technology and not a specific device.

see also What does SFU mean? What is the full form of SFU?