The best person to ask is normally the person setting you the task. But here's my understanding - at least from the perspective of a person who occasionally stays in hotels...
A hotel has a total number of rooms available (say 50). However, guests can book a room for a given date (or series of dates). This is called a reservation i.e. the hotel is guaranteeing the guest will get a room.
However, at this point, the hotel typically does not allocate a specific room. They simply know that they have 50 rooms in total and that they have a number of reservations (say 20) for a given date. So on that date they can take bookings for another 30 rooms.
When the guest arrives at the hotel, the hotel then allocate a room i.e. a specific room/room number. I guess this depends on which ones have been cleaned. Or maybe they have a preference to allocate refurbished rooms first. Also, a hotel my ask if the guest want smoking/non-smoking because many hotels designate rooms this way. Quite often you can be asked if you prefer smoking/non-smoking when you reserve but this is not guaranteed until you arrive and they allocate a room (since there might not be one available).
Note that if the reservation process is working correctly then there should always be at least one room available to allocate when the guest (who has reserved) arrives.
Just to add to the mix, many hotels offer different grades of room e.g. single, twin-bed, double queen etc. Just take a look at the website for a hotel chain. So if the hotel has different grades then the guest is normally able to book a specific grade. Therefore the hotel now has to reserve rooms by grade. But again the allocation of the actual room is usually done when the guest arrives.
Also, note that reservations needs to take account of allocated rooms. For example in two day time we know we have 30 of 50 rooms reserved. However, we have guests who have arrived today who have not reserved who are planning to stay for 1 week. Obviously they are going to take some of the remaining available rooms. Maybe allocated guests must first have a reservation (even if the reservation is doen at the time of arrival by the hotel) - but I'm guessing here - it depends on your design.
hth
Chris
