Calendar entries are stored in the user's IMAP sub folder "Calendar" on the Kolab server. Physically they are represented as multi-part MIME emails with the information stored in vCal format encapsulated in a MIME part.
The KDE Kolab clients store and lookup free-busy lists using a dedicated shared IMAP folder on the Kolab server.
The logic behind the calendar events and their handling is entirely done by the client applications. The server mainly acts as a network attached storage in this regard.
All information is exchanged via multi-part MIME email messages between the KDE Kolab clients. The receiving user decides about every incoming event, note, task list, etc. and depending on the decision the KDE Kolab client application moves the email to the corresponding IMAP folder for further processing.