Once you have created a subsession within a parent session, you can view them in your agenda.

In the agenda view, subsessions are marked by an icon within the parent session. This indicates that there are additional sessions beyond the parent.
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If you hover over the agenda, you'll be able to see a summary of the parent session with its connecting subsession(s).

When rescheduling the parent session by dragging and dropping in within the agenda, the subsession(s) continues to exist within the same time interval as the parent session.