Departments group practitioners for the team view, reports and the booking page. They're labels, not permissions, and have no effect on scheduling.

What a department is for

A department is a label on a practitioner: Physiotherapy, Massage, Podiatry. It groups the team in the Department hierarchy view, is a filter on reports, and sorts practitioners on the booking site. It has no scheduling impact; availability, locations and appointment types are set separately.

Set it

Open the practitioner under Settings, then Users & Practitioners, and under Practitioner configuration find Department. Pick an existing name or type a new one; the list grows as you use it. Save.

The Users and Practitioners page in Department hierarchy view, showing two department headings each listing its practitioners.
The Department hierarchy view with Physiotherapy and Massage departments and their practitioners.

See the whole team

Select View hierarchy on the Users & Practitioners page to see practitioners grouped by department, with anyone not yet in one listed last under No department. To rename a department, change it once under Settings, then Autocomplete Labels, in the Departments list; every practitioner in it follows.