The Roster function is used to determine what times in each diary are shown as available (white) or not available (greyed out). This helps users to quickly identify availability in the diary. Your Roster is also used to determine where and when you are available for appointments booked by clients on your Client Portal.
The effect for users: Roster settings serve as a visual cue. Users are able to book appointments in the "greyed out" (not available) times if needed.
When Client Appointments are created in your diary the Roster is used to set the Location of the appointment. (If you are seeing a client outside of your Rostered hours your diary's "Default Location" will be used. This can be found in Setup > Diaries.)
The effect for your clients: If you have given your clients access to the "Client Login" feature they will only be able to book appointments during times that are marked as "available" in the roster and that have 'Client can book online' ticked. All other times will be "greyed out" and they will not be able to schedule appointments during these periods.
The Roster Override allows you to enter one-off exceptions to the main roster for situations such as Public Holidays and Annual Leave. Availability settings in the Customer Roster override the main roster settings.
TIP: You can edit or delete your Roster Override settings at any time.
If you have more than one location you would create a Standard Weekly Roster linked to each location. This will allow you to reflect your availability at each location. Each location can have a color attached which is reflected in the diary to help you easily identify which location you are working in each day. Read more on this here.