The main Calendar gives you visibility of all bookings and lets you manage your listings. From here, you can see the same reservation details that you would see inside a conversation in Inbox.
The Operations Task calendar also gives visibility of all bookings but lets you manage tasks for your Teams. The reservation details are also displayed similarly here.
The key differences in their functions are:
Calendar lets you set nightly pricing for your properties and manually set their availability by blocking/unblocking selected dates. These changes are pushed to all listings in that property, including those on different platforms.
Task calendar lets you view tasks generated automatically by existing task rules, as well as manually create tasks that are not linked to any reservation.