Can I hide a time-off type from employees so that only I can assign it?
Yes, this is possible. When creating a time-off type, you can enable the option Hide time off for the employee. If this is turned on, the employee will not be able to request that type of time off, or even see it. It can only be assigned by an administrator.
You'll find the option to hide time off from employees in the time-off configuration view (absence or leave) when creating it. Depending on the characteristics you want the time-off type to have, you can choose whether to enable or disable the various configuration toggles.
To do this, go to Configuration >> Calendars >> Create >> [Choose between Absence/Leave] >> Hide time off for the employee.

