This article only applies to VisualForce calendars.
For filter configuration on Lightning calendars, see Calendar Filters.
For more information on the difference between VisualForce and Lightning calendars, see Calendar Differences.
On VisualForce calendars, you can customize how the calendar dimensions (i.e. staff or resources) are filtered. You can create visible filters which can be adjusted by users on the calendar. You can also create hidden filters which are not visible to your users and therefore always apply.
Add your custom checkbox, checkbox formula or lookup fields to each specific calendar's filter
Navigate to the Calendars tab
Select the calendar to which you want to add a filter
On the Related subtab, find the Filterable Fields related list and click New
In the Filterable Name field, fill in the API name of the field on the Resource object that you want to filter on
Click Save
The field will now have been added under filter popup on the corresponding calendar