What an appointment slot is

A slot has an owner, a clinician or a team; a place, a site; a duration, inherited from a grid; and a list of services it accepts. Change any one of those four attributes and you have a different slot.

Why can an appointment shown online not exist in the schedule?

Because they are two views of the same resource, refreshed at different rhythms and sometimes built on different grids. The public view can offer blocks every 10 minutes while the back office runs on 15, and the public catalogue can still list services that site stopped delivering months ago.

The patient pays for that difference: told their appointment was reserved, they reach a front desk where it does not exist. Hence the desk's golden rule, never use the word reserved until the booking is confirmed in the system of record.

A slot that goes by unused does not come back, and that is the real cost of a no-show. Working against true availability means reading the scheduling system, through an integration or without an API at all.