Q: com.apple.calendarserver in file sharing option ?

Mac Mini 10.11.4 with server 5.1

in system's preference -> Sharing -> option

I have this com.apple.calendarserver.

try to uncheck it, with password of ever admin but all failed

searching old discussion, tell me it might have a link with mail service, but is it not activated on mine

unless it has something to do with email alert ?

any idea ?

Thank you