If you need to set some seats aside to not sell publically, you can click New Ticket Hold underneath the ticket types you've allocated.
Give the hold a name, select the session(s) and select the seats. You can choose the seats individually or click-and-drag to select multiple.
If you need the seats to be available to purchase at a certain date and time, add this to the Release Date. If you don't add a Release Date, the seats will not ever be available.