HTC makes specific devices for different carriers and regions, even though most of the variants are packed with the same hardware features or specifications. CID and MID: Carrier ID and Model ID respectively, is what helps HTC to identify which particular carrier or region/country the device was made for. This also helps OTA (over the air) software update servers to know which update to push for your device when they become available.
S-Off means Security Off, and that is exactly what you need if you want absolute control over your HTC device. After unlocking your bootloader, flashing a custom recovery and successfully rooting your device, if you want to hack your device even further, then you would need S-Off.
Rooting your android device is overwriting commands to attain admin privileges or “attain the key to have access to the backroom” of your android device. This key, known as root access will give you the privilege to read and write/alter folders and system partitions which are hidden by default.