C:OS is the only one. You can change the name from OS (operating system) to anything you want. That makes it easier to recognize. Right click on C in File Explorer or Disk Management, choose Properties and change the name from OS. The only rule is no odd characters or symbols.
Which partition should be active partition in windows 8....out these 5 partitions C:OS, pbr image , WINRETOOLS ,DIAGS and ESP??
Active partition in windows 8