Calendar Server creates the calendar ID (calid) for new default calendars from the user ID and the domain name.
For example, John Smith's user ID is jsmith, and his LDAP entry resides in the sesta.com domain. The first time he logs into Calendar Server, the system automatically creates a default calendar with email@example.com as the calid. Each subsequent calendar John Smith creates has a calid with firstname.lastname@example.org: prepended to the calendar name. For example, if John Smith later creates a new calendar named meetings, the calid for the new calendar is email@example.com:meetings.
If a user, group, or resource without a default calendar is listed in the attendee list of an event, the system looks up the uid in LDAP in the event owner's domain as the event owner. If no domain is assigned to the owner, the default domain is assumed. The system constructs a calid by appending the domain to the uid.
If the system can't find the uid in the event owner's domain, it will search any other domains the event owner is allowed to search. For more information, see 11.2 Cross Domain Searching in Calendar Server 6.3 Systems.