after doing your phone S-OFF, you do set follow functions on factory settings

- sim lock
- hboot lock ( this is the bootloader lock, you will be able flash cooked rom after this)
- cid lock (region lock so you can't flash it with other roms, unbrand it, or downgrade it)
- user lock (will be set to factory default)
- pattern lock (will be set to factory default)
- phone usage counter (helps you refurbish the phone)

In their devices, HTC have installed a sort of security check whose level is determined by S-OFF/S-ON. Essentially, this security level is a flag stored on the device’s radio that checks signature images for any firmware before it is allowed to be written to system memory. This hinders using any custom ROMs, splash images, recovery etc., and also restricts access to the NAND flash memory. However, when security level is set to S-OFF, the signature check is bypassed, allowing a user to upload custom firmware images, unsigned boot, recovery, splash and HBOOT images, as well as official firmware that has been modified, this enabling maximum customization of your HTC Android device.
Furthermore, S-OFF also reduces restrictions on accessing the NAND flash memory on the device, allowing all partitions (including /system) to be mounted in write mode while the operating system is booted.