What is C4FM Mode in Amateur Radio?

Yesterday, when I went through the specifications of the new FTX-1F QRP radio from Yaesu announced in the Tokyo Ham Fair, I noted that it has C4FM digital mode in addition to the usual SSB, CW, AM and FM  modes. Today I saw a post on X by North West Fusion Group mentioning about their “Squirrel Net” and one of the world’s largest C4FM network. The time of the net at 7 am UK time will be 11.30 am IST. That prompted a quick read up and hence this post. C4FM is an abbreviation for Continuous Four Level Frequency Modulation, a special type of 4FSK or Four level Frequency Shift Keying, used in conjunction with FDMA or Frequency Division Multiple Access. The channel spacing is 12.5 kHz and data transfer speed is 9.6 kbps, allowing GPS position and ID data to be sent in the same time frame. Error correction is also possible in digital communication. There are repeaters in various global locations with connectivity to the internet. C4FM is restricted to Yaesu radios and is not open source as far as I could understand.