What does C stands for on DS-2CD1023G0E-I?

The (C) is a newer revision of camera or NVR model... there are (D) and (E) revisions now also... each revision has its own firmware...

This is a (C) model camera, but the "System Settings" page does not show the (C), so that doesn't help:

Screenshot 2024-12-18 153634.png

The "Firmware Version Property helps identify the correct firmware sometimes:

Screenshot 2024-12-18 153432.png

If you have the original box the camera came in, the model number there will have the revision letter, if there is one.