A patient drops in between appointments, or it is Friday and you are charting Wednesday's last visit from your notes. There is no booking to start from, and you do not want to make one just to open a chart.
From the patient's page to a chart
Choose New session on the patient's page. A short form asks for the Service, Provider, Location and Date & time. It starts with sensible answers: the provider is you if you are bookable, the service and location are the first in your lists (the only ones, in a new practice), and the time is now. Change the time to chart a past visit.
What Start session does
Pressing Start session books the appointment, then opens the chart editor with an empty draft on it. You are not waiting on a save at the end of charting. The appointment is recorded as completed, because the visit is happening or has happened. It is not a hold on your calendar.
Every session belongs to an appointment, which keeps a visit's date, service and provider in one place instead of in two records that could disagree. The service you pick also decides which chart you get, so a service with its own template opens on that one (a chart for each service).
When the time is taken
The booking runs the same checks as any other, so you are told if it fails. You will see a message if the room is already booked at that time, if it would double-book the provider, or if the provider's connected calendar shows them busy. Nothing is created until you change the details and try again. If you have already booked the visit, use Start session on the Calendar instead, which carries the details into the chart.