Bookmarking this here in forum for future reference. Got to know this when troubleshooting some Calendar issues. Apparently, BES uses some of the hidden system fields for tracking e.g. $CSVersion, $CSFlags etc. Good to know these.

Question
The following table describes each field used in IBM Lotus Notes/Domino Calendaring & Scheduling (C&S) documents, providing field type, use, and valid values as appropriate.

Answer
First, some facts about Form names and their usage:
1: Non-workflow C&S entries (Appointments, Reminders, All Day Events, Anniversaries) use the Form name "Appointment".

2: Workflow C&S entries may have 1 of 2 possible values for the Form name, depending on their current state:
A: If the entry is "in process" (that is, if it is still in the Inbox and has not actually been acted upon), then the value it has is "Notice".
B: If the entry is "processed" (it may be in the Inbox or not but it has already been acted upon, such as Accepting it), then the value is "Appointment".

3: Non-workflow Tasks (that is, Personal Tasks) use the Form name "Task".

4: Workflow tasks (Group Tasks) may have 1 of 2 possible values for Form, depending on their current state:
A: If the entry is "in process" (if it is still in the Inbox and has not actually be acted upon), then the value it has is "TaskNotice".
B: If the entry is "processed" (it may be in the Inbox or not but it has already been acted upon, such as Accepting it), then the value is "Task".

Link: IBM - Notes/Domino Calendaring & Scheduling field descriptions