What does Sony camera a7m4c2 mean?

It is what we usually call a custom shooting mode.

By setting the custom shooting mode, you can set the camera settings such as white balance setting and exposure compensation that are often used in specific scenes to C 1 or C2 and save them. You don't need to make any settings when you encounter the same shooting environment in the future. Just set the camera dial to custom mode, and the photos taken will be the same as those taken after carefully setting the parameters last time.