“Block time slots for pending meetings“ is located in the admin panel at /admin/appointments.
By default toggle this is OFF. When an admin turns the toggle ON, then timeslots with pending/incoming meetings become blocked for users who are trying to create a new meeting.
Users with incoming meeting requests (not accepted and not rejected) also cannot send meeting requests for those timeslots — he sees incoming timeslots as blocked as well.
Pending and incoming meetings are displayed in the schedule in the “Diary activity“ column.