Accommodation options are part of the registration form (they can also be added at a later stage, i.e. conference fee can be paid first, and accommodation later). The number of rooms available at the hotel is limited.
The special room rates for the conference are guaranteed (depending on the availability of the rooms in the hotel) till 30 June. This means that the payment for the rooms needs to be made by this date (reservation alone is not enough). After 30 June, room bookings will be on request and the prices can be different from currently advertised ones.