CS in the name refers to "checksum". All extended messages place a checksum byte in D14. How this is computed or checked... uncertain.
For those out there that are interested, I did a bit of research to try to figure out how to use the PLM to send commands to the devices using the new i2CS. From various online sources I collected a few command traces that use i2CS:
I quickly noticed that the bytes representing the device ID (XXXXXX) are not used to compute the checksum as I was able to send the first command to a new thermostat adapter (using i2CS) and it worked to turn it off.
While I did not figure out how the checksum is computed, I noticed that, from one command to another, the offset in the addition of all the bytes, after the device ID bytes, matches the inverse offset in the checksum. Let us look at the commands 2 and 3 above for an example.
The checksum of command 2 is 0xC2 which is smaller than the checksum of command 3, 0xCA, by 0x08 units. The sum of the extended command bytes in command 2 is 0x10F and for the command 3 is 0x107. The difference between the two sums is again 0x08. But as the difference in sums is positive, the difference in the checksums is negative. This is what I mean by "inverse" offset.
By using this approach, various other commands can be built. These are examples of commands used for the thermostat adapter: