What exactly is Freestyle Dining?
From what I can tell, you have the choice of either traditional "fixed" dining or Freestyling. With Freestyle you can choose to make a reservation each day our eat at a buffet. Most pax may not make a reservation more than 24 hours in advance (suites get 48 hours). Some restaurants are extra.
There is not traditional "fixed" dining on NCL. All the restuarants are Freestyle... dine when you want. Though if you like it is possible to make a reservation for the entire cruise if you find a particular service staff.
Personally, I love the variety of choice in the alternate restaurants... though they do carry a surcharge (which varies by restaurant).
And yes, you can make a reservation 24 hrs in advance in those alternate restuarants, and suite guests get to make reservations 48 hrs. in advance if they choose.