You can create as many off-contract activities as desired. The list will be available for each employees of the company and each company can manage her own list.
Go to : Configuration > Activities section > Off-Contract Activities
Click on Add button to add off-contracts activities to your list.
You can active or disactive a off-contract activity through Statut field. If it is disactive, you unable employees to input time on it : it is not displayed in agenda or timesheet.
You can choose to mark your off-contract activity as "unworked". It means that spent time inputted on this activity will excluded from workdays in staffing rate calculation.