A single user can have multiple calendars. These calendars might be hosted at different locations/services. E.g. a person has a 'personal' and a 'workgroup' calendar in 'I-MATH Zimbra' and an additional on 'Google Calendar'. Different applications (like Zimbra Webinterface, Thunderbird, Apple Calendar, mobile device ...) has to be configured to access all the different calendars.
Calendars might be shared between a group of users and read/write permissions might be delegated.