A booking screen shows an empty hour, but the service needs two staff members and a shared room. A calendar alone does not define availability.
List what a booking consumes
Identify staff, rooms, equipment and preparation time. Decide whether travel, cleanup or a buffer belongs around the appointment. Rules may differ by service and location.
Write the customer journey for a new booking, a change and a cancellation. State when a request becomes confirmed and what happens while staff review it.
Test competing requests
Two people may try to book the same resource at nearly the same time. The system needs a clear way to reserve or reject the slot and explain the outcome. A screen that looked available moments earlier is not enough.
Include time zones where relevant and test dates around clock changes. Payment, reminders and third-party calendars introduce separate dependencies that should be scoped explicitly.
Bring the actual service rules and a few awkward examples to the discussion. A useful booking application turns those policies into an understandable process for customers and staff, rather than beginning with a visual calendar and discovering the business rules after launch.
Try a service that crosses the closing time or consumes a resource shared by another service. These examples expose rules a simple empty-slot display may overlook.