By default the drop down options display as Category, Service, Provider, Location. Depending on the type of business you offer you may want to edit this.
Note: Quantity label can also be controlled from this area. Quantity refers to the number of unites for an individual service. For example, a Class may edit quantity label to "Seats" or an event to "Tickets". The quantity number is controlled when setting up each service.
For example, a Yoga Studio may edit the "Provider" label to "Instructor", "Yogi" or any label they desire.
To manage labels: (Service Setting>Theme>Business Labels Set)
The Labels will be reflected in the Booking interface. For example, see the Yoga Studio examples of selection text from the drop down.